Expression of Stat1Ser727Ala or Stat1Tyr701Phe in U-937 cells inhibits ATRA-induced up-regulation of p27 but not p21. (A) pCIneo, Stat1.2, Tyr701Phe.8, and Ser727Ala.4 cells were incubated with ATRA for 72 hours or left untreated. Total mRNA was prepared and subjected to RPA analysis using the hCC-2 multitemplate probe. The lower panels show quantitation of p21 and p27 mRNA expression, normalized to GAPDH (fold induction). Mean ± SD (n = 3-4). (B) Cells were grown with or without ATRA for 48 hours; lysates were prepared and analyzed by Western blot using antibodies directed against p27.