HER2 overexpression in our particular case was paradoxically a confusing factor when asserting the urothelial nature of the tumor—because of the higher frequency of HER2 overexpression in breast cancer—and a mean to further ensure the identity of primary UC and breast metastasis thanks to the concordance between the immunochemical and molecular features of both lesions. This evidence concerns the gene ERBB2 and breast carcinoma.