CDKN1B and Alzheimer disease: Furthermore, changes in the expression of cell-cycle proteins (cyclins D [34] and B [34]–[36], Cdk4 [37], Cdk1 [38], p27 [39], E2F1 [40], and phosphorylated retinoblastoma protein [41]) have been noted in neurons of post-mortem brain specimens from persons with AD.