The level of hyperphosporylated pRB is maintained after ATRA treatment in cells expressing Stat1Ser727Ala or Stat1Tyr701Phe. U-937 sublines were induced by ATRA for 72 hours, whole cell lysates were prepared, and the levels of total pRb or Ser780-, Ser795-, or Ser807/811-phosphorylated pRb were determined by Western blot analysis using phosphospecific antibodies.