CDK5 and Alzheimer disease: We have shown before that the signal transduction pathways involving the inhibitor of DNA-binding/differentiation protein-1 (Id1), HIF-1α, cyclin-dependent kinases-5 (CDK5), sonic hedgehog, and protein kinase C-delta (PKCδ) may contribute to Aβ-induced cell cycle reentry and neurotoxicity in fully differentiated postmitotic neurons [15,16,206,208,209,210], adding another piece of evidence supporting the detrimental role of HIF-1α in AD under selected circumstances.