ATRA-induced down-regulation of c-Myc is inhibited by expression of Stat1Tyr701Phe or Stat1Ser727Ala. (A) pCIneo, Stat1.2, Ser727Ala.4, and Tyr701Phe.8 cells were induced by ATRA for 8 hours or left untreated. Total RNA was prepared, and the levels of c-Myc and GAPDH mRNAs were determined by Northern blot analysis. Quantitation of c-myc expression, normalized to GAPDH, is shown in the lower panel (fold induction). (B) Stat1.2, Ser727Ala.4, and Tyr701Phe.8 cells were induced as described earlier, and whole cell lysates were prepared. Equal amounts of protein were subjected to immunoprecipitation using α-c-Myc antibodies. The immunocomplexes were collected and analyzed by Western blot analysis using biotin-coupled pan-Myc antibodies and streptavidin-HRP. The arrow indicates an unknown protein that is unspecifically coprecipitated by the α-c-Myc antibodies and serves as an internal control.