The protease activity in FDC-P1 nuclear extracts can be inhibited by PMSF. Both unstimulated (−) or IL-3–stimulated (+) FDC-P1 and FDC-P1(M) cells were lysed in the presence or absence of the protease inhibitor PMSF (P) and nuclear (N) extracts were prepared. Extracts from FDC-P1 and FDC-P1(M) cells were analyzed either separately (lanes 9 through 14) or mixed and incubated for 15 minutes on ice (lanes 1 through 8). STAT5 proteins were detected by Western blotting using anti-STAT5 antibodies (89p).