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If your put oncogenes into mouse cells you get tumours. If you do the same with human cells you get no tumours. Normal mouse cells, and most human tumour cells, express telomerase, and mouse cells have much longer telomeres than human cells. Human cells have been made to form tumours by using chemical or physical agents or an entire viral genome but now, for the first time, it has been done by inserting defined genes, including a telomerase gene, into the cells (William C Hahn and colleagues.Nature1999;400:464–8). Researchers in the USA used retroviruses as vectors to introduce three different genes into human embryonic kidney cells and fibroblasts; they were the hTERT gene, which encodes a subunit of telomerase, an oncogenic allele of H-ras, and a simian virus large T oncoprotein. Cells that expressed all three genes showed tumour-like growth features and produced tumours when injected into nude mice. Cells expressing only two of the genes did not produce tumours. The authors suggest that at least four different biochemical signalling pathways are involved (one more than in mice), one affected by ras, one by hTERT, and one by large T. Maintaining telomere length is probably necessary for human cells to develop into tumours. They do not speculate about the future implications of their research but presumably they are considerable.