TP53 and Alzheimer disease: While there is an increase in the total p53 level, the level of phosphorylated p53-Ser15 decreased along with a decrease in the total p21 and phosphorylated p21-Thr145 level in peripheral blood lymphocytes of patients with AD (Tan & Evin, 2012), suggestive of G1/S check point dysfunction (Zhou & Jia, 2010).