This study, which included 808 patients in first-line treatment of metastatic HER2-positive breast cancer, showed that the addition of pertuzumab led to a statistically significant improvement in progression-free survival (PFS) compared to trastuzumab and chemotherapy alone (from 12.4 to 18.5 months) and to an improvement in median overall survival (OS) from 40.8 to 56.5 months. This evidence concerns the gene ERBB2 and breast cancer.