Breast cancer patients ≤40 years tend to have more triple-negative and fewer luminal A and B breast cancers [17–19], tumors of higher grade, more extensive intraductal component, more lymphovascular invasion, more likely estrogen receptor- (ER-) negative tumors [20–23], and more often BRCA-1 or -2 germline mutations [13, 24–27]. This evidence concerns the gene ESR1 and breast cancer.