Genetic tumours of Xiphophorus are one of the classical experimental models that underline the concept that cancers develop as a result of abnormal gene expression. Formal genetics has indicated that cancer development in Xiphophorus starts when oncogenes are expressed abnormally due to elimination of tumour suppressor genes. The suppressor gene Diff seems to suppress malignancy by controlling terminal differentiation of cells. It appears now that control of terminal differentiation may also be one of the properties of human tumour suppressor loci, in particular the Rb gene. Although it is difficult at this point to envision which molecular or biochemical function of tumour suppressor genes we might be able to identify, research on tumour suppression will at least allow another glimpse at how basic mechanisms of cell differentiation and multiplication operate. It is not clear, however, if elimination of tumour suppressor genes alone is sufficient to elicit the fully malignant phenotype. Cytogenetic studies have shown various nonrandom chromosomal abnormalities in those human tumours in which elimination of a tumour suppressor gene seems to be a critical step in tumorigenesis. In Xiphophorus, it is obvious from our molecular studies that additional genetic events can contribute to the malignant phenotype. Of these, amplification of cellular DNA may have a role in malignant progression of melanomas. At this point, the exact contribution of amplification to genetic melanoma is unclear. Judging from the role of amplification in human and murine tumours, the significance of amplification, in addition to suppressor elimination, in melanomas of Xiphophorus is likely to be high.