Epo dose-response of 32D cells expressing comparable numbers of wild-type (left) and mutant (right) EpoRs. The cells were cultured for 5 days in medium containing IMDM, 10% fetal bovine serum, and the indicated concentrations of Epo (in units per milliliter). The number of viable cells was measured by the trypan blue dye exclusion technique. Each point indicates the mean ± standard deviation of four independent cultures at each indicated Epo concentration.