In support of this, the recent PERTAIN study examining the efficacy of pertuzumab plus trastuzumab and an aromatase inhibitor (anastrozole or letrozole) in patients with HER2-positive and hormone-receptor-positive, metastatic/locally advanced breast cancer met its primary PFS endpoint (median PFS: 18.9 months vs. 15.8 months for trastuzumab alone; stratified HR 0.65; 95% CI 0.48–0.89; P < 0.01) [15]. This evidence concerns the gene ERBB2 and breast cancer.