Bacillus thuringiensis is a naturally occurring, inert soil-dwelling bacterium, commonly used as a biological pesticide (it was registered in 1961). It makes proteins that are toxic to some insects when eaten (caterpillars, butterflies, beetles), but not others. The proteins are not toxic to humans, birds, mammals and most non-target insect species, although Bt resistance became apparent in 1981 (Hilary Wright: 2000, p.68-9). A genetically modified version of Bt was created, by engineering Bt into the permanent genetic code of plants, risking Bt resistance developing in major pests, wild plants acquiring Bt genes through cross-pollination, and Bt produced by transgenic plants differs from the original bacterium by accumulating in the soil (Hilary Wright: 2000, p.69).