While breast cancer is highly heterogeneous, it is broadly categorized into luminal and basal subtypes on the basis of gene expression patterns and classified into three major therapeutic subtypes: estrogen receptor-positive (ER+; receiving endocrine therapy), HER2-positive (HER2+; targeting HER2), and triple-negative (lacking expression of ER, progesterone receptor and HER2; receiving chemotherapy) [27,28]. The gene discussed is ESR1; the disease is breast cancer.