Loss of a whole chromosome 5 or a deletion of the long arm of this chromosome, del(5q), is observed in 10% of patients with a myelodysplastic syndrome (MDS) or acute myeloid leukemia arising de novo, and in 40% of patients with therapy-related MDS or AML (t-MDS/t-AML). To identify a leukemia-related gene on chromosome 5, we previously delineated a 970 kb segment of 5q that is deleted in all patients examined, and prepared a genomic contig and transcript map of this region. Mutation analysis of 20 candidate genes within the commonly deleted segment did not reveal inactivating mutations in the remaining alleles, nor was there evidence of transcriptional silencing via DNA methylation. These observations are compatible with a haploinsufficiency model in which loss of one allele of the relevant gene(s) perturbs cell fate. One candidate gene is EGR1, which encodes a zinc finger transcription factor that is a member of the WT1 family of transcription factors. EGR1 has been shown to regulate hematopoietic cytokine levels (IL3, GM-CSF). Mouse embryo fibroblasts that are null or hemizygous for Egr1 bypass senescence and have apparently immortalized growth characteristics, consistent with loss of a tumor suppressor gene. Subsequent studies revealed that Egr1 is a transcriptional activator of both the p53 and p21Cip1/Waf1 genes during the stress response as well as during senescence, thereby representing a critical mechanism for controlling proliferation, growth arrest, differentiation, and apoptosis. Loss of Egr1 function may allow hematopoietic stem cells to bypass p53-mediated senescence or apoptosis, thereby contributing to leukemogenesis. To evaluate the role of Egr1 in hematopoiesis, we obtained Egr1+/− mice from J. Milbrandt (Washington University). Egr1−/− mice are viable, with mild post-natal growth retardation and infertility. Complete blood counts and body weight were collected for wild-type, Egr1+/−, and Egr1−/− mice every 6 weeks over the course of one year. The Egr1−/− mice display elevated white blood cell counts, elevated lymphocytes, and decreased neutrophil counts, and are unable to maintain normal RBC counts, Hb, and Hct as compared to wild-type and heterozygous mice. To examine the role of Egr1 in erythropoiesis, we evaluated the erythropoietic response to phenylhydrazine-induced hemolytic anemia by treating Egr-1-deficient mice with standard doses of phenylhydrazine (60 mg/kg). Egr1+/− and Egr1−/− mice were unable to respond to the anemia and died with 2 days of the treatment, whereas wild type mice recovered fully within 12 days of the treatment. Egr1-deficient mice treated with N-nitroso-N-ethylurea (ENU), a potent DNA alkylating agent, develop MDS or T cell lymphoma. Egr1-deficient mice developed lymphomas at an increased frequency and rate compared to wild-type animals, indicating that Egr1 cooperates with other mutations in the genesis of hematopoietic neoplasms. MDS was seen only in the Egr1-deficient mice, and is characterized by elevated white blood cell counts, anemia, and thrombocytopenia, with ineffective erythropoiesis in the bone marrow and spleen. Together, these results suggest a role for Egr1 in murine erythropoiesis and implicate EGR1 in the development of myeloid leukemias characterized by abnormalities of chromosome 5.