CDK5 and Alzheimer disease: Similarly, the abnormally hyperphosphorylated tau protein in NFTs is thought to be generated from an imbalance in the kinase/phosphatase system, indicated by a series of activity-altered enzymes involving cyclic AMP (cAMP)-dependent protein kinase/protein kinase A (PKA), cyclin-dependent protein kinase 5 (CDK5), glycogen synthase kinase 3β (GSK3β), protein phosphatase 1 (PP1), and protein phosphatase 2A (PP2A), which are key players in the progression of AD (Wang et al., 2014).