PCNA and Alzheimer disease: A number of laboratories have reported the re-expression of various cell cycle proteins in neurons from patients with AD: cyclins A [52], B [52], D [53], and E [52], as well as CDKs [51], Proliferating Cell Nuclear Antigen (PCNA) [51, 52], Ki67 [52] and cyclin-dependent kinase inhibitors (CKIs) of both the Ink (Inhibitors of Kinases) and Cip/Kip (CDK interacting protein/Kinase inhibitory protein) families [54, 55].