Cell cycle events play a major role in the loss of neurons not only in the advanced stages of AD but are also present in MCI individuals, as shown by the fact that PCNA, cyclin D, cyclin B and Cdk4 expressions are increased in the hippocampus and in the basal nucleus of Meynert from MCI individuals similar to that of AD patients [20, 179]. Here, CDK4 is linked to Alzheimer disease.