Dose-dependent activation of STAT5, Ras, MEKs and Erks by Epo in primary erythroid progenitors (PEPs). Starved (st) PEPs were mock-stimulated (m) or stimulated with various concentrations of Epo for 10 min as indicated. 100 μg of total cell protein was then separated by SDS-PAGE and immunoblotted with anti-P-STAT5, anti-P-MEK1/2, anti-P-Erk1/2 or anti-Erk1/2. GTP-loaded Ras was precipitated with GST-c-Raf1 RBD from 500 μg total cell protein and immunoblotted with anti-Ras. Ø indicates protein extracts from non-starved and non-treated PEPs (cultured in the presence of IL-3, IL-6, SCF and Epo).