ESR1 and breast carcinoma: In breast cancer cells, Skp2 expression is associated with lack of estrogen receptor (ER) and HER-2 [22], which is reflected by the elevated expression levels of Skp2 in the triple-negative breast cancer cell lines MDA-MB-468, and, to a lesser extent, MDA-MB-231, relative to ER-positive MCF-7 cells.