Because of the strong association between Cks1, Skp2, and p27Kip1 levels and the ER status of the tumors, we next examined the expression of these proteins in estrogen-dependent (MCF-7 and T47D) and estrogen-independent (MDA-MB-231) breast carcinoma cell lines, and the possible effects of estrogen modulation on the regulation of Skp2 and Cks1 expression. This evidence concerns the gene ESR1 and breast carcinoma.