ESR1 and breast cancer: This study aimed to investigate the inhibitory effect of P4 on E2-induced cell proliferation, migration, and invasion in Estrogen receptor (ER)+/progesterone receptor (PR)+ breast cancer cells by examining its regulatory role in the epithelial-mesenchymal transition (EMT).<h4>Methods</h4>ER and PR-positive MCF-7 clonal variant (MCF-7 CV) breast cancer cells were treated with E2 and co-treated with various concentrations of P4.