Co-immunoprecipitation assay demonstrated that treatment with P4 increased the complex formations of cSrc-SHP2, cSrc-PRA, cSrc-PRB, and cSrc-cavolin-1, PR-SHP2, and PR-caveolin-1, and the levels of p-PRA, p-PRB, and p-cSrcY416, but did not significantly affect the complex formations of cSrc and cSrc-negative proteins (Csk and p140Cap) in breast cancer cell lines, and the levels of p-Csk, p-caveolin-1, and p-cSrcY547 (inactive form of cSrc). This evidence concerns the gene SRC and breast cancer.