In order to determine if the pro-proliferative effect of RARγ in the Myc-expressing mouse mammary tumor cells is also present in human mammary cells, RARγ was knocked-down with siRNAs in immortalized non-tumorigenic human MCF-10A cells, estrogen receptor (ER+) MCF-7 cells and triple negative (ER-/progesterone receptor-/HER2-) MDA-MB-231 breast cancer cells. This evidence concerns the gene ESR1 and breast cancer.