These findings hold additional interest in light of the recently published TREnd trial,40 a phase 2 clinical study which assessed the activity of palbociclib as monotherapy, as well as in combination with the endocrine therapy (aromatase inhibitor or fulvestrant) upon which the patient had experienced previous progression as a prior line of therapy for ER+/HER2neg metastatic BC. This evidence concerns the gene ESR1 and breast cancer.