In line with the previous studies, the reported multicenter experience has retrospectively evaluated activity, duration of response, progression free survival, safety and clinical and pathological factors potentially influencing these outcomes in a series of patients with metastatic HER2 positive breast cancer treated with second-line T-DM1 following pertuzumab, trastuzumab and docetaxel in first-line. This evidence concerns the gene ERBB2 and breast carcinoma.