IGF1 and breast carcinoma: Notably, the study on breast cancer cells found that stimulation of KCC4 mRNA levels by insulin-like growth factor-1 (IGF-1), another serum growth factor, was inhibited by 65% with an inhibitor of ERK1/2, PD98059, but was insensitive to the p38 MAPK inhibitor, SB202190 [66].