Considering that CDKN1B is mutated predominantly in luminal breast cancers [46], we analyzed the phenotypic effects (i.e., alterations of cell proliferation, cell cycle, and motility) associated with p27 HRV overexpression in two breast cancer cell models, the (ER+PR+/-HER2−) MCF-7, and the triple-negative MDA-MB-231 cell lines. The gene discussed is ESR1; the disease is breast cancer.