CDK5 and head and neck cancer: CDK5 or its activators P35 and P39 are reported to be hyper-expressed in many cancers, such as breast cancers, head and neck cancers, colon cancers, lung cancers, prostate cancers, yolk sac tumour, seminoma cancers [25,52–54], while hypo-expressed in brain and central nervous system (CNS) cancers, leukaemia cancers and liver cancers [27].