In the autumn of 2000 a graduate student from the University of California held a workshop for local peasant farmers in the beautiful mountainous region of Sierra Norte de Oaxaca in southern Mexico. The graduate, David Quist, hoped to show the farmers how to test their seeds for GM. To do this he thought he would show them the difference in the purity of the local maize, called criollo, compared to the maize that had been shipped in from the USA, where some 40 per cent is GM.