CDK5 and Alzheimer disease: Such aberrant hyperphosphorylation of NFs in AD is likely mediated by abnormal hyperactivity of kinases such as Erk1 and 2 (Veeranna et al., 2004), Cdk5 (Shea et al., 2004), and GSK3β (Chen et al., 2005), or the decreased activity of phosphatases such as PP-2A and 1 (Rudrabhatla et al., 2009).