ERBB2 and breast cancer: Prior to our study, Goel et al.13 and Ciruelos et al.14 assessed the use of continuous low-dose ribociclib or a three-week regimen of 200 mg palbociclib combined with trastuzumab in HER2-positive BC including patients with HR-negative disease, respectively, but both studies showed limited activity, especially in HR-negative patients.