After IMiD compounds were pulse treated for less than 4 hours and washed, followed by 14 days of colony formation in culture, the number of BFU-E colonies was significantly decreased ( .05), whereas that of colony-forming unit granulocyte (CFU-G) colonies NOD-IN-1 was significantly increased ( .05; supplemental Number 3), demonstrating the potent effect of IMiD compounds within the induction and growth of myeloid lineages. resulted in resistance to POM-induced IKZF1 downregulation and reversed the POM-induced lineage shift in colony-formation assays, suggesting the POM-induced degradation of IKZF1 in CD34+ cells requires CRBN. Chromatin immunoprecipitation assays exposed that IKZF1 binds to the promoter region of PU.1, suggesting that PU.1 is a direct downstream target of IKZF1 in CD34+ cells. POM failed to induce IKZF1 degradation in IKZF1-Q146H-OE CD34+ cells, indicating that CRBN binding to IKZF1 and subsequent IKZF1 ubiquitination is critical in this process. Using the NOD/SCID/-c KO mouse model, we confirmed the induction of myeloid progenitor cells by IMiD compounds at the expense of common lymphoid progenitors. These results demonstrate a novel mechanism of action of IMiD compounds in hematopoietic progenitor cells, leading to selective degradation of transcription factors critical for myeloid maturation, and clarify the event of neutropenia associated with treatment by IMiD compounds. Visual Abstract Open in a separate window Intro Sixty years ago, thalidomide was used to treat nausea, particularly morning sickness in pregnant women, but it was consequently banned because of its teratogenicity. Since then, thalidomide derivatives including lenalidomide (LEN) and pomalidomide (POM) have been developed and utilized for the treatment of multiple NOD-IN-1 myeloma (MM), resulting in significantly improved overall survival in individuals with myeloma.1-5 Immunomodulatory drug (IMiD) compounds inhibit myeloma cell growth, block cytokine production, impair angiogenesis, and enhance T-cell stimulation and proliferation, leading to MM cell death.6 Recently, IMiD compounds were shown to bind to cereblon (CRBN), the substrate acknowledgement component of cullin-dependent ubiquitin ligases. Treatment of zebrafish with thalidomide results in fin problems, suggesting that IMiD compounds take action by stabilizing CRBN substrates.7-9 In MM, LEN treatment leads to the selective ubiquitination and degradation of 2 lymphoid transcription factors, IKZF1 and IKZF3, from the CRBN-CRL4 ubiquitin ligase.10-12 IKZF1, also known as Ikaros, is a zinc finger-containing DNA-binding protein that takes on a pivotal part in immune homeostasis via transcriptional rules of the earliest phases of lymphocyte ontogeny and differentiation.13 Functional deficiency of IKZF1 has been implicated in the pathogenesis of acute lymphoblastic leukemia, the most common form of child years malignancy.13 This finding raises a concern because long-term follow-up data suggest that IMiD compounds are associated with an increased risk for secondary hematologic malignancies. In particular, patients receiving melphalan plus LEN regimens experienced a significantly higher risk of developing a second main malignancy than those who did not get LEN (risk percentage, 4.41; 95% confidence interval, 2.4-8.1; .0001).14 Interestingly, previous studies have shown that in hematopoietic progenitor cells (HPCs), IMiD compounds do not show direct stem cell toxicity; instead, these compounds impact lineage commitment and induce cell growth.15,16 Treatment with pomalidomide during the development of NOD-IN-1 primary human being erythroid cells induces the suppression of several known repressors of fetal globin gene expression.17 We have previously shown that IMiD compounds shift hematopoietic lineage commitment to myeloid colony formation at the expense of erythroid cell colony formation by downregulation of GATA1.18,19 Other major adverse effects of IMiD compounds are thrombocytopenia (grade 3/4, 14.7%) and neutropenia (grade 3/4, 41.5%), which often compromise optimal treatment with IMiD compounds.20,21 Our previous studies have shown that IMiD compounds downregulate ER81 PU.1, a NOD-IN-1 critical transcription element for myeloid maturation, leading to the maturational arrest of granulocytes, the build up of immature myeloid precursors, and subsequent neutropenia.16 However, the precise mechanism involved has not yet been identified. Here, we statement that CRBN is definitely expressed in human being CD34+ cells and that POM promotes the CRBN-dependent degradation of the IKZF1 protein in CD34+ cells. Knockdown of CRBN.