The use of cyclin-dependent kinase 4 and 6 inhibitors (CDK4/6 inhibitors) (palbociclib, ribociclib, and abemaciclib) in combination with endocrine therapy is the current standard of care for the first- or second-line treatment of hormone receptor-positive, HER2-negative metastatic breast cancer [1,2,3]; also, abemaciclib and ribociclib plus endocrine therapy have demonstrated an improvement in invasive disease-free survival in patients with HR+/HER2- early breast cancer whose tumors had a high risk of recurrence [4,5]. This evidence concerns the gene ERBB2 and breast cancer.