Levels of β-globin mRNA before and after stimulation with Epo in G418^r clones of Ba/F3 cells from two independent transfections. (Top panel) G418^r clones (designated G-1 through G-4) maintained in IL-3–containing medium (IL-3 0 hours) were washed and exposed to either Epo alone for 3 hours, 9 hours, 1 day, and 2 days or to Epo and IL-3 for 2 days. Total cell RNA was hybridized with β^major-globin and actin probes. (Bottom panel) G418^r clones (designated E-1 through E-10) and a mixture of G418^r cells propagated in suspension culture (designated E-m) derived from a separate transfection were washed and exposed to Epo for 2 days. The β-globin mRNA contents before and after Epo exposure were measured.