To confirm whether such scenario is really the case duringspontaneous cell immortalization, we tested during the process ofMEF-immortalization, (1) because MEFs are immortalized with the mutation in theArf/p53 module similar to cancer development [17], (2) becauseprimary MEFs often develop tetraploidy prior to immortalization, and (3) becausesenescing cells are known to spontaneously accumulate unrepairable DNA lesions[18], as potentially precancerous DNA lesions. This evidence concerns the gene TP53 and cancer.