ESR1 and breast cancer: In the follow-up TBCRC023 trial, which included 97 patients with HER2+ BC and compared a 12-week vs. a 24-week regimen of trastuzumab plus lapatinib (with letrozole—plus LHRHa if premenopausal—if ER+), the pCR rates after 24 weeks of neoadjuvant treatment were 12.1% vs. 27.9% at 12 vs. 24 weeks in the overall population [90].