CDK5 and Alzheimer disease: Research study has given an indication that Cyclin D, Cdk4, Cdk5, mitotic signaling G-protein Ras, PCNA (Proliferating cell nuclear antigen), a S phase marker, have been found to be up-regulated in neuronal cells of AD brains, due to which these cells exit the G0 phase and re-enter the cell cycle (Zhu et al., 2004).