Using swordtail-platyfish hybrids, scientists found a proto-oncogene – a normal gene that causes cancer when turned on – that causes spontaneous melanoma formation in these fish. This gene, XMRK, resides on the sex chromosome and allows melanoma to be inherited. The importance of studying XMRK in swordtail-platyfish models is the universal commonalities among proto-oncogenes in different organisms, meaning a similar gene with similar inheritance patterns could very well be what causes melanoma in humans.