NTRK1 and melanoma: Crosses between species of the fish genus Xiphophorus: X. maculatus (the southern platyfish) and X. hellerii (the green swordtail) produce fish that spontaneously develop malignant melanoma induced by a proto-oncogene encoding a receptor tyrosine kinase designated Xiphophorus melanoma receptor kinase (Xmrk), which is located on the sex chromosome [6].