Amplification of the erythroid lineage in the spleen of ERK1^−/− mice. CFU-Es were measured after 2 days, and total CFCs, BFU-Es, and CFU-GMs were measured after 7 days in complete methylcellulose with cytokines. Data are mean colony numbers ± SEMs from 5 ERK1^+/+ and 6 ERK1^−/− (***P < .001).