MDM4 and melanoma: The loss of the regulatory function of p53 (the transcribed protein of TP53) is a late event in melanomas and is caused by several mechanisms, including structural mutations in TP53; the overexpression of the mouse double minute 2 (MDM2) oncoprotein, which is responsible for the ubiquitination and degradation of p53; mutations in P14ARF, which lead to an overexpression of MDM2; and the overexpression of the MDM2 homolog murine double minute x (MDMX) which inhibits p53 [124,125].