However, the p53-signaling pathway is inoperative in almost all types of human cancer; factors that inactivate p53 specifically include genetic mutations or deletions (Feki and Irminger-Finger, 2004), defective post-translational modifications, and interactions with its main endogenous inhibitors, MDM2 (Momand et al., 1998) and MDMX (Shvarts et al., 1996). The gene discussed is MDM4; the disease is cancer.