A novel biosensor has been developed which can detect deoxyribonucleic acid (DNA) damaging chemicals. Such genotoxic chemicals can cause cancer in humans. The biosensor presents advantages over the existing Ames test (which merely measures survival rates in bacteria). It is amenable to continuous, high throughput screening. The organism at the heart of the biosensor, yeast, possesses similar DNA-damage response systems to human cells. It is a generic reporter: it can be adapted to detect other types of damage, such as oxidative stress, with high specificity. The output signal, fluorescence, is readily measurable in living cells and is reagent free. The biosensor can be employed in water supply monitoring (genotoxicity is a legislative issue for polluters and water supply companies) and for compound screening (no dangerous drugs or additives are involved).