Different breast cancer subtypes are commonly diagnosed based on the histological expression of three receptor proteins: estrogen receptor (ESR1, also known as ERα), progesterone receptor (PGR), and human epidermal growth factor receptor 2 (HER2, also known as ERBB2), and by the differential expression of fifty select genes (PAM50) which infer the “intrinsic” subtype. This evidence concerns the gene ESR1 and breast cancer.