The GM Science Review Panel and other expert bodies recommend that GM crops that produce medical or industrial substances should be designed and grown in ways that would avoid gene flow to food and feed crops (GM Science Review Panel). The monarch butterfly controversy (Box 24 on page 71) demonstrated that it is difficult to extrapolate from laboratory studies to field conditions. Field studies have shown some differences in soil microbial community structure between Bt and non-Bt crops, but these are within the normal range of variation found between cultivars of the same crop and do not provide convincing evidence that Bt crops could be damaging to soil health in the long term (GM Science Review Panel).