CDK6 protein contains a catalytic core composed of a serine/threonine domain.This protein also contains an ATP-binding pocket, inhibitory and activating phosphorylation sites, a PSTAIRE-like cyclin-binding domain and an activating T-loop motif. CDK6 is involved in a positive feedback loop that activates transcription factors through a reaction cascade. CDK6 is important for the control of G1 to S phase transition. However, in recent years, new evidence proved that the presence of CDK6 is not essential for proliferation in every cell type, the cell cycle has a complex circuitry of regulation and the role of CDK6 might be more important in certain cell types than in others, where CDK4 or CDK2 can act as protein kinases compensating its role.