CDK5 and Alzheimer disease: Threonine residue 668 (T668) is phosphorylated in vivo by protein kinases glycogen synthase kinase-3β (GSK3β), stress-activated protein kinase 1b (SAPK1b)/c-Jun N-terminal kinase 3 (JNK3), cell division cycle 2 (Cdc2), and cyclin-dependent kinase 5 (Cdk5), and phospho-T668-APP was found upregulated in the hippocampus of AD brains.