ESR1 and breast cancer: Triple-negative BC, defined as BC that is negative for estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2), is another subgroup of interest since recent observations have suggested that this subtype of BC is more frequent in less developed countries (17% to 28%), notably in Africa [19]–[24], compared with Europe and North America, where the proportion of triple-negative BC is about 10–12% [25], [26].