Transfected U-937 cell lines constitutively express exogenous Stat1 proteins.

(A) The levels of Stat1 protein in U-937 stably transfected clones were determined by Western blot with α-Stat1 antibodies. (B) To ensure that the high Stat1 levels were maintained during ATRA-induced differentiation, cells were induced by ATRA for 72 hours and analyzed by Western blot using α-Stat1 antibodies. (C) Immunoprecipitation with antibodies against the hemagglutinin tag 12CA5 followed by Western blot using α-Stat1 antibodies shows exogenous expression of Stat1HA in established cell lines.