PCNA and Cognitive impairment: Hippocampal neurons in mild cognitive impairment (MCI) patients express proliferating cell nuclear antigen (PCNA), cyclin D and cyclin B [15], and CKIs such as p16 (INK4a), p18 (INK4c) and p27 (KIP1) are expressed in neurons of AD patients [129, 130], indicating the integral involvement of the cell cycle machinery in affected neurons.