p53 is required for DNA damage induced apoptosis, which is central to its function as a tumor suppressor. Here we show the apoptotic defect of p53 deficient cells is nearly completely rescued by inactivation of any of the three subunits of the DNA repair holoenzyme DNA-PK. Intestinal crypt cells from p53 nullizygous mice were resistant to radiation induced apoptosis, while apoptosis in DNAPKcs/p53, Ku80/p53, and Ku70/p53 double null mice was quantitatively equivalent to that seen in wild type mice. This p53 independent apoptotic response was specific to the loss of DNA-PK, as it was not seen in Ligase IV/p53 or ATM/p53 double null mice. Further, it was associated with increased phospho-Chk2, and cleaved caspases 3 and 9, the latter indicating engagement of the intrinsic apoptotic pathway. This demonstrates there are two separate, but equally effective, apoptotic responses to DNA damage, one is p53 dependent and the other, engaged in the absence of DNA-PK, does not require p53.