The metal beryllium is essential for making nuclear weapons, among other things, but exposure to even small quantities can lead to chronic beryllium disease, a serious or even fatal lung condition. Genetic markers have been found which indicate an unusually high sensitivity to beryllium. Should the laboratory test for these, and advise those who test positive to move to different jobs? Or will this reduce the incentive to cut beryllium exposure for everyone else, or to monitor all workers for early signs of the disease? Some people told they have an increased genetic risk for heart disease respond by working hard to minimise other risk factors - through exercise or healthy eating. The evidence that some genetic tests may turn out to be a distinctly mixed blessing is an argument for regarding each new testing programme as an experiment, and monitoring outcomes carefully.