However, in many cancer types, such as leukemia, sarcoma, and melanoma, genetic mutations of TP53 are less common, and p53 inactivation is often caused by alterations in p53-regulatory proteins, in particular by amplification and overexpression of MDM2 (HDM2 in humans) and MDMX (HDM4 or HDMX in humans) [1,3,4,5,6,7]. The gene discussed is MDM4; the disease is leukemia.