Expression of full-length and truncated STAT5 proteins in hematopoietic cell lines. (A) Nuclear extracts were prepared from unstimulated (−) or IL-3–stimulated (+) A4, 15S, FDC-P1 and FDC-P1(M) cells. Full-length (STAT5α) or truncated (STAT5β) STAT5 proteins were detected by Western blot analysis using monoclonal anti-STAT5 antibodies (89p). (B) FDC-P1 and FDC-P1(M) cells were grown in IL-3–containing medium (Med) or starved for 20 hours (−) and stimulated with IL-3 (+) for 30 minutes. Nuclear extracts were prepared and incubated with multimerized β-casein oligonucleotides coupled to Sepharose beads. Bound proteins were eluted and analyzed by Western blotting with anti-STAT5 antibodies (89p). (C) A labeled DNA probe containing the β-casein GAS element was added to nuclear extracts from IL-3–stimulated FDC-P1 and FDC-P1(M) cells and subsequently incubated either without (−) or with antibodies directed against the C-terminus (C) or N-terminus (N) of STAT5. Complexes were analyzed by an EMSA.