The clinical subtypes are based on the expression of the estrogen receptor (ER) and the progesterone receptor (PR) and the amplification of the HER2 (ERBB2) receptor and include hormone receptor (HR)-positive (HR+/HER2−, also ER+/PR+/HER2−), triple-positive (HR+/HER2+, also ER+/PR+/HER2+), HER2-positive (ER−/PR−/HER2+, also HER2+), and triple-negative breast cancer (TNBC) (HR−/HER2−, also ER−/PR−/HER2−) [2,3,4]. The gene discussed is ESR1; the disease is triple-negative breast carcinoma.