Since we previously demonstrated that P4 (0-100 nM) concentration-dependently promoted the proliferation of breast cancer cell lines (T47D and MCF-7), and reached a plateau at 50 nM (9) that overlaps the physiologic range of plasma P4 concentrations in premenopausal women (22), we used P4 at 50 nM to investigate the role of SHP2 in the P4-induced cSrc activation. The gene discussed is SRC; the disease is breast cancer.