A western blot analysis of U2OS human osteosarcoma cells that have wild-type p53, and of SAOS2 cells that have mutant non-functional p53, demonstrated that CHEK1 is activated to a higher extent in the p53 mutant background than in the p53 wild-type background (Figure 4B) validating this prediction. The gene discussed is TP53; the disease is osteosarcoma.