This entails that most of the CDKs and cyclins tend to be altered in HNSCC, which may be caused by either overexpression of cyclins (mainly D1 and E1) and/or CDKs (mainly CDK4 and CDK6), as well as loss of CKI (mainly INK4A, INK4B and KIP1) and RB expression [34]. Here, CDK4 is linked to head and neck squamous cell carcinoma.