Core-binding factor acute myeloid leukemia (CBF-AML) is associated with KIT mutations and deregulated expression of KIT.

The contribution of clonal hematopoiesis (CH) and disease-initiating precursors in B-cell non-Hodgkin lymphomas (B-NHLs) remains underexplored. Such precursors may drive clonal evolution, contributing to disease progression and relapse. Here, we systematically profiled genetic precursor lesions in 43 B-NHL patients using complementary whole exome, targeted, and single-cell sequencing approaches. CH-associated mutations with a variant allele frequency of ≥1% were detected in the peripheral blood of 55% of patients, with significantly higher frequencies in indolent compared to aggressive B-NHL (p=0.03). Quantification of allele burden in flow-sorted cell populations revealed a B-cell-skewed expansion of CH clones - contrasting the myeloid differentiation bias reported in individuals without hematologic malignancies. Gene-specific expansion patterns were evident among the most frequent CH lesions, with DNMT3A-mutant clones exhibiting impaired hematopoietic differentiation and TET2-mutant clones showing multi-lineage propagation. Notably, identical CH clones were detected in 41% of corresponding lymphomas, displaying distinct clonal dynamics: tumor-promoting CH (expansion in B-NHL; 10/16 clones; mainly TP53) and tumor-infiltrating CH (no expansion; mainly DNMT3A). Moreover, we identified lymphoma-associated mutations in flow-sorted hematopoietic progenitors from patients with indolent but not aggressive B-NHL and observed a stepwise accumulation of mutations along the lymphoid differentiation path. Single-cell genotyping confirmed the presence of mutated progenitors in 3 follicular, 2 mantle cell and 2 marginal zone lymphoma patients, providing direct evidence of a pre-neoplastic state in disease pathogenesis. Our findings offer novel insight into the cellular origin of nodal B-NHLs and highlight a previously underappreciated role for early clonal events involving the stem/progenitor cell compartment.

Recessively inherited loss-of-function mutations in Excision Repair Cross-Complementing 6 like 2 (ERCC6L2) cause a bone marrow failure (BMF) syndrome characterized by moderate cytopenias, frequent somatic TP53 mutations, and a propensity to develop myeloid malignancies. The pathophysiology and molecular mechanisms underlying the BMF syndrome as well as its association with TP53-mutant clonal hematopoiesis (CH) and myeloid malignancies have remained poorly understood. Using novel preclinical in vitro and in vivo model systems, we demonstrate that Ercc6l2 maintains the competitive fitness of hematopoietic stem and progenitor cells (HSPCs) by mitigating replication stress. Sustained replication stress and DNA damage in Ercc6l2-deficient HSPCs cause p53 pathway activation followed by cell cycle arrest and apoptosis. Moreover, Ercc6l2 deficiency results in decreased expression of master hematopoietic regulators Runx1 and Gata1 in HSPCs. Altogether, loss of Ercc6l2 leads to reduced HSPC numbers, bone marrow hypocellularity, and cytopenias. Notably, somatic Trp53 mutations restore cellular fitness of Ercc6l2-deficient HSPCs by abrogating p53 pathway activation and restoring Runx1 and Gata1 expression, thereby correcting the BMF phenotype. However, p53 loss fails to normalize replication stress, allowing for the accumulation of DNA damage over time which increases the likelihood for leukemic transformation. Our data uncover the pathogenesis of ERCC6L2 disease and provide a prototypic example of clonal compensation in BMF syndromes, where somatic mutations in leukemia-associated genes - in this case TP53 - transiently improve blood cell production at, however, the expense of increasing leukemogenic potential.

Chronic active Epstein-Barr virus (EBV; CAEBV) disease is an uncommon, often lethal T and/or natural killer (T/NK)-cell lymphoproliferative disorder that remains underrecognized outside Asia. Recent advances in molecular and immunopathologic studies, together with the 2022 International Consensus Classification and the fifth World Health Organization lymphoma classification, have consolidated the disease concept and diagnostic framework. Recent studies support a model in which mutated EBV infects hematopoietic stem or lymphoid progenitor cells in the bone marrow, establishing latent infection that persists as these progenitors differentiate into T/NK cells. The infected lymphocytes subsequently undergo clonal expansion, immune evasion, and progressive accumulation of genetic and epigenetic alterations, giving rise to systemic CAEBV disease and, in some cases, transformation into EBV-positive lymphomas or leukemias. We review the clinical spectrum and differential diagnosis in relation to EBV-associated hemophagocytic lymphohistiocytosis and EBV-positive lymphomas or leukemias and highlight geographic differences between Asian and non-Asian cohorts. Despite progress, diagnosis remains hampered by the lack of standardized and commercially available assays to identify infected cell subsets. Hematopoietic stem cell transplantation remains the only curative option; however, transplant-related mortality, relapse, and suboptimal outcomes in adult-onset disease underscore the need for optimized conditioning and pretransplant disease control. We review emerging therapeutic strategies, including programmed cell death 1 blockade, JAK inhibition, and EBV-specific cytotoxic T-lymphocyte therapy, and outline priorities for prospective international trials. This review aims to raise global awareness among hematologists and foster collaborative studies to improve outcomes for patients with CAEBV disease.

Patients with T-cell lymphomas and leukemias have overall poor outcomes due to the lack of targeted and effective treatments, particularly in the relapsed and refractory settings. Development of chimeric antigen receptor (CAR) T-cells against T-cell neoplasms is limited by a lack of discriminating T-cell antigens that allow for effective anti-tumor responses while preventing CAR T-cell fratricide. We hypothesized that targeting CD2, a pan-T-cell antigen, using anti-CD2 CAR T-cells engineered without CD2 expression (CART2), would support CAR T-cell manufacturability and preclinical efficacy. Optimized CD2-knockout CART2, generated using CRISPR-Cas9, eradicated primary patient-derived CD2+ hematological neoplasms in vitro and in vivo, secreted effector cytokines, and exhibited adequate proliferative capacity. Nevertheless, CD2 has a key costimulatory function, and its deletion could lead to CAR T-cell dysfunction. Therefore, we tested the role of the CD2:CD58 axis in CAR T-cells, using the anti-CD19 CART models. We demonstrate that CD2 loss attenuates CART19 efficacy by reducing avidity for tumor antigen, co-stimulation, and ultimately in vivo activity. Analogously, we show that tumor CD58 loss reduces CART19 efficacy. To overcome this issue, we developed a novel PD-1:CD2 switch receptor that rescues intracellular CD2 signaling, particularly when PD-L1 is engaged, resulting in improved in vivo outcomes. Collectively, we studied the role of CD2 both as a target for CAR T cell therapy and as a critical costimulatory protein, whose signaling can be rescued using the PD-1:CD2 switch receptor. This receptor can be incorporated into CAR T-cells and provides an effective strategy to overcome CD2-signaling deficiencies.

Circadian rhythms orchestrate immune activation and effector function, yet whether within-day timing influences chimeric antigen receptor (CAR) T-cell therapy outcomes remains unknown. We conducted an international, multicenter retrospective study of 1052 adults with relapsed or refractory large B-cell lymphoma treated with CD19-directed CAR T-cell therapy across 7 centers (2017-2025). The median infusion time was 11:48 am (interquartile range, 11:06 am to 12:45 pm). Each hour later in infusion time was associated with an increased risk of progression, relapse, or death (hazard ratio, 1.11; 95% confidence interval, 1.03-1.20; P = .004) after adjustment for center, product, and key clinical variables. One-year progression-free survival (PFS) was 51.4% for early (before 12:00 noon) infusion vs 35.2% for late (at or after 12:00 noon) infusion, whereas overall survival was similar between groups. The PFS benefit was driven by lower relapse and higher complete response rates in the early infusion group. Although no differences were observed in immune toxicities, late infusion correlated with higher peak inflammatory markers and reduced day 7 CAR T-cell expansion. Together, these findings suggest that the timing of CAR T-cell infusion may influence therapeutic efficacy and support prospective evaluation of circadian-informed delivery strategies.

A single, universally used tumor classification system is critical to ensure precision in patient care, clinical trial enrollment, and advancing therapy. Since the publication of the third edition of the World Health Organization (WHO) classification in 2001, the hematology and hematopathology community has benefited from a single, widely accepted classification system. However, in 2022, 2 separate classification systems of hematologic and lymphoid neoplasms emerged, namely the fifth edition of the WHO Classification of Haematolymphoid Tumours and the International Consensus Classification. This has created confusion and has led to a global call to action to return to a single classification system. In 2024, leaders of the Society for Hematopathology (SH) and the European Association for Haematopathology (EA4HP), together with the leadership of the WHO Classification of Tumours, developed a plan to bridge the division and unite around the next edition of the WHO Classification of Haematologic and Lymphoid Tumours (WHO6). This plan entails convening a classification advancement meeting (CAM), organized by the SH and EA4HP, with broad participation by pathologists, clinicians, and geneticists to discuss and debate updates and changes to the existing classifications. Published conclusions of the CAM meeting will be available for independent consideration by the editorial board of WHO6. We are convinced that engagement with the broader community and the leveraging of global expertise to unite around WHO6 will critically benefit patients, medicine, and science.

Multiple myeloma (MM) with t(11;14)(CCND1;IGH) remains the only subset sensitive to the BCL2 inhibitor venetoclax. However, not all patients with t(11;14)(CCND1;IGH) respond to treatment, and some progress early after initial response. To investigate this, we examined 44 whole-genome and whole-exome sequencing data samples from 34 patients with t(11;14) MM treated with venetoclax. The presence of mutations in the RAS pathway was strongly associated with shortened progression-free survival (PFS) and was validated in an independent cohort of 21 patients with MM. The presence of 1q gain was also associated with shorter PFS in patients without RAS mutations. In 10 patients with paired prevenetoclax and postvenetoclax treatment samples, postvenetoclax progression was recurrently driven by the selection of genomic events in the BCL2/MCL1 and RAS pathways and of high-risk features (eg, loss of TP53 and CDKN2C). Overall, our study shows that comprehensive genomic profiling can identify most mechanisms underlying resistance to BCL2 inhibition in t(11;14)(CCND1;IGH) MM.

Sickle cell nephropathy (SCN) is a major clinical complication in sickle cell disease (SCD), yet its underlying mechanisms remain incompletely defined. Hemolysis, a hallmark of SCD, has been implicated in SCN pathogenesis, but the downstream inflammatory pathways are not fully understood. We previously demonstrated that hemolysis triggers type I interferon (IFN-I) responses, leading to the upregulation of the C-C motif chemokine ligand 2 (CCL2) and recruitment of classical monocytes that differentiate into monocyte-derived macrophages (MoMϕ) within livers in SCD. In this study, we show that IFN-I and CCL2 levels are elevated in the plasma of patients with SCD with abnormal urine albumin-to-creatinine ratio and in the kidneys of the SCD Townes mouse model. Using IFN-I receptor (Ifnar1)-/- and CCL2 receptor (Ccr2)-/- mouse models of SCD, we demonstrate that the loss of IFN-I or CCL2 signaling reduces MoMϕ accumulation, renal inflammation, and renal injury. Mechanistically, we identify that hemin-induced IFN-I production occurs via the Toll-like receptor 3 (TLR3)/TIR-domain-containing adapter-inducing interferon-β (TRIF) signaling axis, independent of MyD88, MAVS, or STING. These findings uncover a previously unrecognized heme-TLR3/TRIF-IFN-I-CCL2 pathway that contributes to renal pathology in SCD and suggest that targeting this axis may offer therapeutic benefit.

Somatic frameshift mutations in the gene encoding calreticulin (CALR) give rise to myelofibrosis and are classified as type 1 (del52) or type 2 (ins5) according to the degree of wild-type sequence retained adjacent to the neopeptide, with each type conferring different clinical outcomes. Targeting strategies specific for type 1 vs type 2 mutations would have enormous clinical utility in the treatment and prevention of myelofibrosis as responses to tyrosine kinase inhibitors are not durable nor mutation specific. Here, we show that dual targeting of type 1 (del52) mutant CALR with 2 monoclonal antibodies directed against distinct epitopes in CALR have significant advantages compared with single-agent treatment in the eradication of primary megakaryocyte progenitors in vitro and in a humanized ossicle microenvironment leading to improved survival in xenograft models. Dual targeting was superior in blocking constitutive STAT5 and extracellular signal-regulated kinase phosphorylation induced by del52 and prevented accumulation of Janus kinase 2 (JAK2) phosphorylation, overcoming ruxolitinib resistance. In contrast, type 2 mutations showed increased CALR dimerization and were partially resistant to antibody targeting but could be affected by a ruxolitinib triple combination. Together, our data demonstrate an ultraprecision medicine approach tailored to either type 1 or type 2 mutation classes will be required for maximal efficacy and complete blockade of JAK/STAT signaling, with far-reaching implications for patient management.

We hypothesized that transcriptomic features among disease-driving hematopoietic stem and progenitor cells (HSPC) could improve the current paradigm for risk stratification in myelofibrosis (MF). Therefore, we performed bulk RNA-seq on blood from 358 MF patients split into training and test cohorts (ClinicalTrials.gov Identifier: NCT02760238). Single-cell (sc) RNA-seq data from Lin-CD34+ MF HSPCs were used to guide the development of a prognostic model trained on the bulk RNA-seq data. A NanoString assay was used to validate our final model in two independent cohorts of MF patients. We identified a 24-gene weighted-sum expression score (termed MPN24) that was prognostic of overall survival (OS). In two independent cohorts of 170 and 100 MF patients, MPN24-High scoring patients had worse 5-year OS compared to MPN24-Low scoring patients (25.5% vs 86.9%, HR = 9.81, P = 4.0e-11 and 23.5% vs 75.9%, HR = 6.40, P = 1.8e-7, respectively). MPN24-High was also associated with clinical, mutation and karyotypic features known to confer adverse risk in MF. The MPN24 score retained independent prognostic significance in multivariable analysis incorporating these covariates as well as existing risk stratification models including DIPSS, DIPSS Plus, MIPSS70, and MIPSS70 Plus v2.0, adding significant prognostic value to these risk stratification models (P = 1.6e-5, 4.0e-6, 1.5e-6, 5.4e-4, respectively). The MPN24 score was particularly useful in improving risk-stratification of DIPSS-Intermediate, and MIPSS70 Intermediate and High-risk patients. MPN24 is an HSPC-derived gene expression score that is associated with overall survival independent of clinical and genomic prognostic variables and improves risk stratification in MF.

Heatstroke, a severe hyperthermic condition, is characterized by a core body temperature exceeding 40℃ and multiple organ dysfunction syndrome (MODS) with an extremely high mortality rate. Despite advances in identifying heatstroke-induced cell death pathways, the molecular cascades that bridge heat-induced cell death to MODS and mortality are not yet fully characterized. Our findings demonstrate that Z-DNA binding protein 1 (ZBP1)-triggered disseminated intravascular coagulation critically drives MODS and fatal outcomes in heatstroke. Heat stress activates ZBP1-dependent necroptosis, promoting tissue factor (TF) release and phosphatidylserine (PS) externalization. Genetic knockout of ZBP1 or its downstream necroptotic effectors, reduction of global TF expression, suppression of PS exposure, or pharmacological inhibition of the coagulation cascade attenuates heat stress-induced coagulation activation, organ injury, and death. Comparable results are obtained in heat-stressed mice with conditional knockout of ZBP1 in hematopoietic or myeloid lineages. Overall, our study reveals the critical role of ZBP1-mediated necroptosis in bridging heat stress and coagulation dysfunction.

Pediatric hemophagocytic lymphohistiocytosis (HLH) is a life-threatening hyperinflammatory syndrome, characterized by heightened T-cell activation and overwhelming interferon gamma production. Rather than a binary "primary" vs "secondary" framework, it is best conceptualized as a threshold phenomenon that occurs when combined genetic and environmental factors exceed a critical tipping point, leading to a recognizable clinical pattern of abnormal immunopathology. In addition to classic genetic cytotoxic defects, an increasing number of genetic lesions continue to be recognized as related to HLH. Environmental triggers frequently include malignancy, infection, and rheumatologic disorders, and identifying underlying factors driving HLH is a crucial component of evaluation. The revised HLH-2004 criteria are an important tool in promptly recognizing HLH, but clinical criteria may be fulfilled by other inflammatory disorders. Genetic and functional immune testing are complementary tools for diagnosis and establishing recurrence risk. Dexamethasone and etoposide remain cornerstones of initial therapy but are best used in an individualized, response-based manner with close monitoring of inflammatory markers. Newer, more targeted agents continue to emerge and are often critical additions for achieving disease control. Most patients with inherited cytotoxic defects and other select causes require hematopoietic cell transplant for cure, which should occur as soon as feasible once adequate, not perfect, HLH control is established to minimize risk of reactivation, infection, and toxicity. This review aims to highlight key advancements in defining, diagnosing, and treating HLH based on a growing understanding of HLH pathophysiology. We also provide perspectives on practical clinical approaches that may be useful for diagnosing and treating pediatric HLH.

The hematopoietic system performs essential functions for living organisms through a wide array of cell types, including immune cells for host defense, erythrocytes for oxygen transport and megakaryocytes involved in vascular and tissue repair. The adoptive transfer of mature blood cells and the transplantation of hematopoietic stem cells (HSCs) have been extensively exploited in clinical settings, for the treatment of inherited blood disorders as well as blood malignancies. In addition, blood cells are also preferential targets in the context of gene therapy, where they provide revolutionary treatments not only for classical hematologic diseases but also for novel and unexpected indications, such as inherited metabolic disorders. However, clinical bottlenecks remain, including the limited availability of suitably matched donor cells, the need for high cell doses, and the complexity of personalized manufacturing.Given their proliferative ability and capacity to generate any human cell type, human pluripotent stem cells (hPSCs), encompassing both embryonic stem cells (ESCs) and induced PSCs (iPSCs), represent a novel, potentially unlimited, easy-to-engineer source of either patient-specific or allogeneic blood cells for "off-the-shelf" cell therapies. In parallel, hPSC-derived blood cells enable in vitro modeling of diverse hematologic diseases and the identification of novel therapeutic targets. Recent advances describing protocols for the de novo generation of HSCs are opening new and exciting avenues for a broad application of hPSC-derived blood cells in both basic research and clinical medicine.

Most patients with early-stage classical Hodgkin lymphoma (cHL) are treated with doxorubicin, bleomycin, vinblastine, and dacarbazine with or without radiation therapy, although studies are now evaluating the incorporation of novel agents paired with abbreviated chemotherapy. We present the efficacy and safety of brentuximab vedotin (BV) and nivolumab in combination with doxorubicin and dacarbazine (AN+AD) in patients with early-stage cHL. In this phase 2 study, patients with non-bulky (<10 cm) Ann Arbor stage I or II cHL received 4 cycles of AN+AD. The primary endpoint was complete response (CR) rate at end of treatment (EOT) by investigator. At the time of this analysis, 154 patients received ≥1 dose of AN+AD. The objective response rate at EOT was 96% (95% CI, 91.7-98.6), and the CR rate was 92% (95% CI, 86.0-95.4). In the favorable (n=56) and unfavorable (n=97) subgroups, CR rates were 95% (95% CI, 85.1-98.9) and 91% (95% CI, 83.1-95.7), respectively. The proportion of patients with duration of CR of at least 2 years was 96% (95% CI, 90.9-98.4). At a median follow-up of 27.9 months, the estimated 2-year PFS rate was 97% (95% CI, 92.0-98.8). Any-grade and grade ≥3 treatment-emergent adverse events occurred in 99% and 44% of patients, respectively; no events of febrile neutropenia were reported. Any-grade treatment-emergent immune-mediated adverse events occurred in 22% of patients. One disease-related death was reported after the safety reporting period. Results from this study support the use of BV and nivolumab in combination with limited chemotherapy in patients with non-bulky, early-stage cHL. ClinicalTrials.gov: NCT03646123.

Intestinal Enterococcus domination has been associated with an increased risk of mortality from acute graft-versus-host disease (GVHD) after allogeneic hematopoietic cell transplantation (allo-HCT), a curative-intent treatment for patients with hematologic malignancies. In this study, we investigated interactions between Enterococcus and the intestinal epithelium as a mechanism to aggravate GVHD. We observed that endogenous intestinal Enterococcus outgrowth was associated with increased GVHD mortality and major histocompatibility complex class II (MHC-II) expression by intestinal epithelial cells in the colon in an MHC-disparate mouse model of GVHD. Monocolonization of nontransplanted gnotobiotic mice with Enterococcus faecalis was sufficient to induce colonic MHC-II expression. Conversely, select species within the genus Enterococcus, as well as a consortium of 4 anaerobic commensal bacteria including Blautia producta, did not affect colonic MHC-II expression in gnotobiotic mice. In addition, E faecalis colonization induced inflammatory responses in CD4+ T cells and natural killer cells from the colonic lamina propria, the 2 main sources of interferon gamma production that drives MHC-II expression in nonprofessional antigen-presenting cells. We further explored the potential therapeutic benefit of establishing colonization resistance against E faecalis through administration of a lantibiotic-producing B producta strain after allo-HCT. Colonization of transplanted mice with a consortium of commensal bacteria containing the lantibiotic-producing B producta strain prevented intestinal Enterococcus domination after transplantation and improved GVHD survival. Our results demonstrate a potential mechanism by which Enterococcus aggravates GVHD through increased MHC-II expression in the intestinal epithelium. Targeting the Enterococcus-epithelium-MHC-II axis thus presents a therapeutic opportunity to prevent lethal GVHD.

NUTM1 rearrangement defines a significant subset of B-cell acute lymphoblastic leukemia (B-ALL), particularly in infants lacking KMT2A rearrangements, yet its underlying molecular characteristics remain poorly understood. Here, we establish that NUTM1-rearranged (NUTM1-r) leukemia is a discrete entity characterized by a unique transcriptional and epigenetic landscape, notably featuring global DNA hypomethylation, irrespective of the 5' fusion partner. Functional interrogation of NUTM1 fusions reveals a dual oncogenic role: they drive commitment toward the B-lymphoid lineage while concurrently conferring potent leukemic stem cell properties. Strikingly, expression of a representative fusion, BRD9-NUTM1, is sufficient to induce serially transplantable B-cell progenitor, prepro-B-like leukemia in vivo, faithfully recapitulating the key molecular and immunophenotypic features of human NUTM1-r B-ALL. Mechanistically, NUTM1 fusions establish an aberrant chromatin state, marked by global enhancement of H3K27 acetylation and the creation of distinctive open chromatin regions that co-opt both B-lineage and stemness-related transcriptional programs, including those involving NF-κB and posterior HoxA genes. In stark contrast to resistant KMT2A-rearranged leukemias, NUTM1-r leukemic cells exhibit a profound sensitivity to chemotherapy. This vulnerability is mechanistically linked to the leukemia's dependence on active transcription. Our findings delineate the unique molecular profile of NUTM1-r leukemias, revealing specific vulnerabilities that rationalize their favorable clinical outcomes and suggest opportunities for modified therapeutic strategies.

High levels of circulating tumor cells (CTC) are a powerful predictor of poor outcomes in newly diagnosed multiple myeloma, yet the mechanistic underpinnings of this correlation remain unknown. To investigate whether CTC-related pathobiology is driven by a specific CTC subset, paired bone marrow and blood samples from patients with newly diagnosed multiple myeloma were analyzed by single-cell transcriptomics and whole-genome sequencing. This revealed that down to the individual clone level, CTC and paired bone marrow cells are transcriptionally similar, without evidence for a distinct circulating population. In contrast, bone marrow myeloma cells from patients with high CTC levels had increased proliferation and unbalanced primary genetic events, including enrichment for MAF and CCND translocations. To investigate the impact of heterogenic genomic events on CTC levels, whole-exome and bulk RNA sequencing from the Multiple Myeloma Research Foundation CoMMpass data set were analyzed and validated in our in-house data sets. Bone marrow tumor cells from patients with high CTC levels were uniformly characterized by transcriptomic signatures of proliferation. Furthermore, CTC levels were uniquely dependent on primary genomic events and high-risk secondary genomic events, including amplification 1q, deletion 1p, deletion 13q, biallelic TP53 mutations, and increased apolipoprotein B editing complex-induced mutations even in patients without MAF translocations. Finally, we developed a model that predicts the impact of genetic alterations and tumor burden on CTC levels. In sum, we reveal that CTC are the net result of tumor burden, primary translocations, and secondary genomic events, making CTC a powerful biomarker for genomics-driven high-risk disease in patients with newly diagnosed myeloma.

The use of posttransplant cyclophosphamide was initially pioneered as a means of permitting haploidentical transplantation across HLA barriers. This approach has now become a standard of care for the prevention of significant acute and chronic graft-versus-host disease (GVHD) after related and unrelated HLA-matched allogeneic peripheral blood stem cell transplant across a full spectrum of conditioning intensities. This article discusses recent advances, the mechanisms of action, and important unresolved questions in the prevention of GVHD that will help inform new prospective clinical studies.

Sickle cell disease (SCD) and β-thalassemia are devastating genetic disorders resulting from defects in the β-globin subunit of adult hemoglobin. Both disorders are ameliorated by the induction of γ-globin, a component of fetal hemoglobin (HbF). Therefore, the development of safe, effective, and widely available inducers of HbF is needed. Here, we discovered that slow cycling erythroid cells exhibit increased γ-globin expression. To understand the molecular basis of this, we screened all cyclin-dependent kinase inhibitors (CDKIs) for their ability to induce HbF using CRISPR activation. We found that overexpression of CDKN1B, which encodes p27Kip1 (but not overexpression of other CDKIs), induces γ-globin expression at the transcriptional level. CDKN1B mutants expressing proteins unable to bind/inhibit CDKs and/or cyclins revealed that γ-globin induction by p27Kip1 depends largely on the domains involved in its cell cycle function. Pharmacological inhibition and genetic reduction of CDK4/6 also result in increased HbF. In genetic rescue experiments, we show that p27Kip1 induces HbF by inhibiting CDK4/6, through a mechanism that is likely BCL11A and ZBTB7A independent. Furthermore, palbociclib, an oral CDK4/6 inhibitor, significantly increases HbF in a murine SCD model at doses that are well tolerated. Moreover, we show that HbF induction by hydroxyurea, a drug currently in use to treat SCD, may be mediated in part by CDK4/6 inhibition. Overall, our findings establish a causal relationship between CDK4/6 activity and γ-globin production and suggest that single or dual CDK4/6 inhibitors might be therapeutically beneficial for SCD and β-thalassemia.