In early mammalian erythropoiesis, histone methyltransferase Dot1l plays a critical role in controlling the number of circulating erythroid and myeloid cells, as indicated by Dot1l-mutant mice that developed more slowly and died between E10.5 and E13.5, displaying profound anemia, which was especially apparent in the small vessels of the yolk sac. The gene discussed is DOT1L; the disease is anemia (phenotype).