Smad2, Smad3, and Smad4 complex with N-CoR through Ski. 293T cells were transfected with Flag–Smad2, Flag–Smad3, or Flag–Smad4 either alone (lanes 1–3) or together with HA–Ski (lanes 4–6). Endogenous N-CoR complexed with the Smads was isolated by immunoprecipitation with an anti-Flag mAb and detected by Western blotting with an anti-N-CoR antibody. Cell lysates were also blotted directly with anti-Flag and anti-HA antibodies for control of Smads and Ski expression. As a positive control, N-CoR associated with HA–Ski was isolated by immunoprecipitation with an anti-HA mAb and blotted with an anti-N-CoR antibody (lane 7). (Lane 8) HA-tagged mutant Ski(241–323) isolated from transfected 293T cells by immunoprecipitation with an anti-HA antibody. (Lane 9) Anti-Flag immunoprecipitate prepared from cells cotransfected with Flag–Smad3 and HA–Ski(241–323).