MAP2K1 and anemia (phenotype): To verify whether the anemia in 1Mek1 and 1Mek2 resulted from a defect in a specific hematopoietic progenitor population, colony-forming assays in methylcellulose were performed with bone marrow and spleen from 10-week-old wt, Mek1–/–, 1Mek1, and 1Mek2 mice.