In HER2CLIMB, patients with metastatic HER2 + BC who were previously treated with trastuzumab, pertuzumab, and ado-trastuzumab emtansine were randomized in a 2:1 ratio to receive trastuzumab, capecitabine, and tucatinib vs trastuzumab, capecitabine plus placebo. This evidence concerns the gene ERBB2 and breast cancer.