Crops vary in their tendency to outcross, and the ability of a crop to outcross depends on the presence of sexually compatible wild relatives or crops. If a resulting transgenic/wild hybrid had some competitive advantage over the wild population it could persist in the environment and potentially disrupt the ecosystem. Research is needed to improve the assessment of the environmental consequences of gene flow, particularly in the long run, and to understand better the gene flow between the major food crops and land races in centres of diversity (ICSU, GM Science Review Panel).