MAP2K2 and anemia (phenotype): To determine whether anemia and the reduced spleen size seen in Mekhema null pups were due to defective expansion or differentiation of specific hematopoietic progenitors, we measured the number of colonies of BFU-E and CFU-GEMM by performing methylcellulose colony-forming assays with AGM at E10.5 and fetal liver at E14.5 from control (Mek2–/– and 1Mek1) and Mekhema null embryos (Figure 2).