Phosphorylation of STAT proteins in HUT78 cells. The cells were treated with (+) or without (−) IFNα (10,000 U/mL for 5 minutes) and whole cell extracts were prepared for immunoprecipitation with anti-STAT antibody. The immunocomplex was then analyzed by Western blot using anti-phosphotyrosine (anti-pY) antibody (upper panel). The same blot was stripped and reprobed with anti-STAT antibody to determine the levels of protein in each lane (lower panel). STAT1 immunoprecipitation is shown in Figure 6A, STAT2 in Figure 6B, and STAT3 in Figure 6C. The arrow in Figure 6B indicates the location of STAT2 and activated STAT1 (doublet) was coprecipitated with STAT2 in HUT78S cells only (Fig 6B, upper panel).