CDK4/6 inhibitors, i.e., palbociclib, ribociclib and abemaciclib, were proven to be effective in treating HR-positive/HER2-negative (HR+/HER2−) breast cancer [19], i.e., for cases where the cancer cells have receptors for estrogen and/or progesterone (HR+) but do not have high levels of the HER2 protein (HER2−). This evidence concerns the gene ERBB2 and breast cancer.