Enhanced BMP4 but not EPOR signaling in ERK1^−/− spleen. (A) Lysates (100 μg) of ERK1^+/+ and ERK1^−/− splenocytes were resolved in sodium dodecyl sulfate–polyacrylamide gel electrophoresis and transferred to a nitrocellulose membrane. Expression and phosphorylation levels of STAT5, STAT1, and ERK1/2 and expression of EPOR were analyzed. (B) Spleen cells from wild-type or ERK1^−/− mice were plated in methylcellulose media containing either EPO (3 U/mL) alone or EPO plus BMP4 (15 ng/mL), and stress BFU-E colonies were counted 5 days later. Data are mean colony numbers ± SEMs from 4 mice per genotype. (C) Q-PCR analysis of BMP4 expression in spleen of ERK1^+/+ (n = 5) and ERK1^−/− (n = 6) mice. Data are mean fold ± SEM (*P ≤ .05).