If the major function of the p53 protein is to function as a DNA damage checkpoint protein, then it is reasonable to hypothesize that its inactivation in human cancer must be related to its DNA damage checkpoint function. This hypothesis further implies that in tumor cells one or more of the DNA damage checkpoint pathways has been activated. Otherwise, p53 would not be active and there would be no selective pressure for TP53 mutations. I make the argument that tumorigenic transformation is intrinsically associated with formation of DNA DSBs in every cell cycle leading to activation of DNA damage checkpoint pathways. In turn, activation of the DNA DSB checkpoint provides the selective pressure for the high frequency of p53 inactivation in human cancer.