Given that both SNU-503 and SNU-503R80Gy grew as irregular convex aggregates in which cells at the center of the aggregates may experience hypoxic damage, a decreased cleaved PARP and active form of the caspase-3 in the induced radio-resistance rectal cancer cells may imply that the mechanism of radio-resistance is associated with resistance to cellular hypoxia. Here, CASP3 is linked to rectal cancer.