STAT1C decreases the expression level of STAT3 and phospho-STAT3 and increase STAT1-STAT3 heterodimer formation. Western blot studies showed that gene transfection of STAT1C induced an appreciable decrease in total STAT3 and phospho-STAT3 in EC1 and EC109 cells. Cell lysates were harvested 48 hours after gene transfection (A). In contrast, both STAT3 and phospho-STAT3 were increased in response to siRNA knockdown of STAT1 in KYSE150 and KYSE510 (B). Co-immunoprecipitation experiments revealed that gene transfection of STAT1C into EC1 and EC109 induced a substantial increase in STAT1:STAT3 heterodimer; at the same time, the total STAT3 protein level was decreased (C). Cell lysates were prepared 48 hours after gene transfection. (E.V.: empty vector).