For example, palbociclib (formerly known as PD-0332991), a novel oral selective inhibitor of CDK4/6 that blocks tumor cell progression, has received a “breakthrough therapy designation” from the FDA for the treatment of patients with breast cancer, and similar drugs, like dinaciclib (SCH727965/targeting CDK1,2,5,9), alvocidib (flavopiridol/ CDK1,2,4,6) and seliciclib/CYC202 (roscovitine/CDK2,5) are at various phases of clinical testing [14]. Here, CDK1 is linked to breast cancer.