CDK4 and breast cancer: On the other hand, ET and in particular Tam is the mainstay treatment for ERα-positive primary BC, especially the LumA ones [2, 4, 5], while CDK4/CDK6 inhibitors (i.e., palbociclib—Palbo and abemaciclib—Abe) are co-adjuvant drugs for the treatment of MBC expressing the ERα [1, 2, 4].