Since canine mammary gland tumors are very common, with most tumors being potentially malignant, and the standard therapy is radical mastectomy with no effective chemotherapy protocols for the treatment of animals with metastatic disease [26], this study aimed to evaluate the antitumor effect of lapatinib on HER2+ and HER2− primary and metastatic canine mammary gland carcinoma cells cultured in vitro. The gene discussed is ERBB2; the disease is metastatic neoplasm.