Effects of ATRA and IFNα on the tyrosine phosphorylation state of Stat 1α in NB4 cells. Cell lysates from NB4 cells unstimulated and stimulated with ATRA (10⁻⁶ mol/L) or IFNα (1,000 U/mL) for the indicated amount of time were immunoprecipitated with anti-Stat 1α/β antibodies. Two equivalent aliquots (30 μL) of the the various immmunoprecipitates (200 μL) were subjected to Western blot analysis in parallel and the corresponding filters were probed with antiphosphotyrosine (upper panel) and anti-Stat1α (lower panel) antibodies, respectively. The molecular weight of Stat1α is indicated on the left. Ip, immunoprecipitatory antibody; WB, Western blot antibody. A representative experiment out of three independent repeats is depicted.