The human mammary carcinoma used as control for HER2 demonstrated a strong complete membrane staining in all the tumoral cells (HerceptTest 3+) while in feline mammary carcinomas, immunoreactive products were seen in the membrane and the cytoplasm of neoplastic cells and, according to HerceptTest method, only complete membrane staining was considered positive. Here, ERBB2 is linked to breast carcinoma.