MDM2 and melanoma: In two of the zebrafish melanoma models published so far [8], [5] it has been shown that tumor formations occurs only in the presence of inactivating mutations in tp53. To check whether tumor formation in our model is also due to an intervening mutation in tp53, we evaluated the ability of p53 to signal transcriptionally following X-ray treatment of kita-GFP-RAS or control fish by looking at the expression of two p53 downstream targets, p21 and mdm2 [24].