ESR1 and triple-negative breast carcinoma: However, younger patients in these countries have a higher proportion of more aggressive basal-like tumors (also known as triple-negative breast cancer; TNBC) that are ER−, progesterone-receptor-negative (PR−), and HER2-negative (HER2−), as well as a higher proportion of HER2-overexpressing tumors (that are ER−/PR−) than older patients (11–13).