(A) Activation of β-casein probe-binding proteins in NBM at different time points. Cells were obtained and extracted immediately (lane 1) or after incubation with Epo for 5 (lane 2), 15 (lane 3), 30 (lane 4), 60 (lane 5), or 180 (lane 6) minutes, or after incubation for 15 minutes with IL-3 (lane 8). Nuclear extracts of equivalent amounts of NBM cells incubated with the β-casein probe were run on a nondenaturing polyacrylamide gel. Arrow indicates position of STAT5 complex. (B) Identification of STAT-binding proteins activated by Epo in NBM cells. Nuclear extracts obtained from nonstimulated cells (lane 1) or from cells stimulated for 15 minutes with Epo (lanes 2 through 5) were incubated with the β-casein probe in the absence (lanes 1 and 2) or presence (lanes 3 through 5) of α-STAT5 antibodies. α5-1 (lane 3), α-sheep STAT5; α5-2 (lane 4), α-human STAT5A; α5-3 (lane 5), α-human STAT5B. Lower arrow indicates position of STAT5 complex, higher arrow the position of antibody-containing complexes.