CDK4 and head and neck squamous cell carcinoma: CDKN2A binds to CDK4 and CDK6 and suppresses proliferation by inhibiting cells progressing from G1 into S phase (Liggett and Sidransky 1998), and has also been recognized as an early event in the progression of pre-malignant lesions leading to HNSCC (Schwarz et al. 2008).