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In the 1920s, the American biologist Dr. Myron Gordon and German biologists Haussler and Kosswig independently discovered that inter-species hybrids of a particular strain of the platyfish, Xiphophorus maculatus, and the swordtail, Xiphophorus helleri, developed cancers virtually identical to malignant melanomas in man (reviewed here). They traced the origin of these tumors to pigment cells of a platyfish color pattern consisting of black spots on the dorsal fin. Genetic studies demonstrated that melanomas developed only in hybrids that had replaced both copies of a platyfish regulatory gene with swordtail forms that could not control proliferation of the platyfish pigment cells (reviewed here). This animal model was one of the first to prove that some cancers were inherited diseases; after 65 years, these fish still are used in cancer research in the United States, Germany, Canada and Japan.
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For more information, please see
http://www.xiphophorus.txstate.edu/about/introduction.html