In the in vitro AD model, we found that Aβ elicited cell cycle reentry in the postmitotic neurons, as revealed by the expressions of cyclin D1 and phosphorylated retinoblastoma protein (pRb-Pi), two G1-phase markers that include proliferating cell nuclear antigen (PCNA) and the incorporation of 5-bromo-2′-deoxyuridine (BrdU) into newly synthesized DNA, together with histone H3 phosphorylated at Ser-10 as the G2/M marker. The gene discussed is PCNA; the disease is Alzheimer disease.