Recent results from the SUMMIT trial, a randomized phase II trial that enrolled 45 patients, showed an overall response rate of 35% and a median PFS of 8.2 months in patients with HR+/HER2− advanced BC with an activating HER2 mutation and prior CDK4/6 inhibitor treatment treated with trastuzumab–neratinib and fulvestrant [52]. This evidence concerns the gene CDK4 and breast cancer.