Those studying tumour cells have no way of knowing all the different mutations that make them divide uncontrollably. Robert Weinberg and colleagues at the Massachusetts Institute of Technology, USA, have successfully transformed a healthy human cell into a cancer cell by making it express three genes that they know are switched on in tumours. "No one has ever been able to make a human cancer cell with a defined set of genes before,' says Weinberg, who hopes that the research will help figure out how mutations interact to govern a tumourous growth ( New Scientist , Vol 163, No 2197).