The STAT5 N-domain is required for STAT5-mediated mast cell survival and bcl-2 expression. (A) STAT5ab^null/null mast cells transduced with MSCV-based bicistronic retrovirus expressing either vector control (IR) wild-type STAT5a or STAT5aΔN were cultured in the indicated concentrations of either IL-3 or SCF for 16 to 24 hours, and compared with wild-type mast cells. Apoptosis was assessed by the presence of subdiploid DNA after Hoechst 33342 DNA staining. Each line in the plot represents an independent experiment (IL-3 withdrawal: IR-GFP vs STAT5a, P < .01; IR-GFP vs STAT5aΔN, P = .64; STAT5a vs STAT5aΔN, P < .01; SCF withdrawal: IR-GFP vs STAT5a, P < .01; IR-GFP vs STAT5aΔN, P = .87; STAT5a vs STAT5aΔN, P < .01; Student t test for all data points in each line). (B) Mast cells as in panel A were cultured in either IL-3 (5 ng/mL), SCF (50 ng/mL), or both IL-3 (5 ng/mL) plus SCF (50 ng/mL) for 48 hours, and total cell lysates were subjected to Western blot analysis for STAT5, bcl-2, and bcl-X_L, and actin expression on the same membrane.