Profiling of in vitro IR response in ALL by Western blotting identifies apoptosis-sensitive and apoptosis-resistant tumors. Left panel shows a representative IR-sensitive ALL exhibiting gradual cleavage of PARP1 and disappearance of full-size PARP1 protein by 24 hours after 5 Gy IR. This is accompanied by up-regulation of p53 protein and its target p21 between 4 and 8 hours after IR, cleavage and disappearance of full-length proteins procaspase-9 and -7, and occurrence of the cleaved caspase-3 by 24 hours after IR. Right panel shows a representative IR-resistant ALL, exhibiting absence of cleavage of PARP1 and procaspase-9, -7, and -3 by 24 hours after 5 Gy IR despite abundant up-regulation of proteins p53 and p21 between 4 and 8 hours after IR. Expression of actin shows equal protein loading. The same Western blot for either representative IR-sensitive or IR-resistant ALL was reprobed with different antibodies multiple times. For each of the tumors, each horizontal group of lanes represents probing with a single antibody, and these individual probings are separated by black boxes.