Although initial observations by Rodríguez et al. [27] identified 17-β estradiol as an enhancer of HLA-I (human MHC class I) expression in ER+ (MCF-7) but not in ER− (MDA-MB-231 and MDA-MB-435s) BC cell lines, the underlying mechanism has yet to be identified. This evidence concerns the gene ESR1 and breast cancer.