Erythroid progenitor cells respond to Epo, SCF, IGF-1, and insulin. (A) Erythroid progenitor cells were analyzed for specific responses to Epo, SCF, GM-CSF, IGF-1, insulin, IL-3, and EGF by Western blotting and staining with phosphotyrosine-specific monoclonal antibody (4G10). One hundred micrograms of protein lysate per lane. Control, no factor added. Cells at day 9 of culture are shown. (B)³H-thymidine incorporation in response to factor of the same cell preparation shown in (A). Factors were applied individually or in various combinations as indicated. ³H-thymidine incorporation was determined 48 hours after the addition of factors; for details see Materials and Methods.