Evidence suggests that HER2-low expression is not exclusive to breast cancer, as recent data reveals its presence in breast (47.1%), gastric/GEJ (34.6%), salivary gland (50.0%), lung (46.9%), endometrial (46.5%), urothelial (46.0%), and gallbladder (45.5%) cancer [103]. This evidence concerns the gene ERBB2 and breast cancer.