Beyond breast cancer, HER2 alterations, including overexpression and activating mutations, have been identified in other cancer types, such as gastric and gastroesophageal cancers (4.4–53.4%) [45,46], non-small-cell lung cancer (4%) [47,48,49,50], endometrial cancer (20–40%) [51,52], and ovarian cancers [53]. This evidence concerns the gene ERBB2 and breast cancer.