One of the main concerns about genetically modified crops is the risk of contaminating 'natural' populations with the introduced genes. A technical development that reduces this risk is now reported, and the principle is simple ? rather than introducing transgenes into the nuclear genome, from which they can be spread in the form of pollen, the genes are introduced into the chloroplast DNA. Chloroplasts are maternally inherited, so wild relatives of transgenic plants cannot be contaminated.