Clinically, breast cancer is stratified by estrogen and progesterone receptor expression and by HER2 status into three clinically relevant groups—HR+/HER2− (luminal, ~70%), HER2+ (~15–20%), and triple-negative HR−/HER2− (~15%)—which exhibit distinct biology, natural history, and therapeutic sensitivity [2]. This evidence concerns the gene ERBB2 and breast carcinoma.