Evidence on the involvement of KOR in breast cancer EMT provided study on human MDA-MB-231 and MCF-7 cells with opioid receptor κ gene (OPRK1) siRNA-downregulated KOR expression, where more pronounced changes were observed in the KOR-deficient breast cancer cells with increased E-cadherin but decreased N-cadherin, SNAIL, vimentin, and phosphorylated AKT kinase at protein levels in comparison to control cell cultures [64]. This evidence concerns the gene OPRK1 and breast cancer.