An engineered xylS-controlled asd gene was introduced into cells along with the xylS-gef system, so that benzoate depletion caused both production of Gef and deactivation of the growth-promoting gene asd. Recently, interleukin 10-secreting auxotrophic Lactococcus lactis (Steidler et al., 2003) has been used to treat Crohn's Disease (Braat et al., 2006). The gene discussed is ARSD; the disease is Crohn disease.