Organophosphorus pesticides, at environmentally relevant concentrations that do not inhibit acetylcholinesterase (AChE), may precipitate asthma in humans (Bryant 1985) and cause airway hyperreactivity, a key feature of asthma, in guinea pigs (Fryer et al. 2004; Lein and Fryer 2005). This evidence concerns the gene ACHE and asthma.