Palbociclib (Ibrance: Pfizer, New York, NY, USA) was the first selective cyclin-dependent kinase (CDK) 4/6 inhibitor approved in Canada for use in combination with letrozole for the treatment of post-menopausal women with estrogen receptor-positive (ER+) HER2– ABC as an initial endocrine-based therapy for their metastatic disease [8], and later in combination with fulvestrant for the treatment of women with HR+/HER2– locally advanced or MBC whose disease progressed after prior endocrine therapy [9]. This evidence concerns the gene ESR1 and metastatic neoplasm.