ERBB2 and breast cancer: Evidence on the topic under debate comes from three recent retrospective studies showing lower response rate and shorter survival outcomes in patients treated with T-DM1 following pertuzumab administration [15, 16, 29] In a previous retrospective multicentric study from our group involving 250 pretreated HER2 positive BC patients we showed lower efficacy of T-DM1 in the pertuzumab-pretreated cohort in comparison with the trastuzumab pretreated group [15].