The human body has a defense system that protects it from the harmful effects of chemical substances. It is believed, however, that chemical substances exhibit their toxicity when this defense system is exposed to substances that overwhelm its capacity, or when the system's capacity has declined. Genetic modification is a powerful tool for researching this defense system. For example, it can identify biomolecules that become the targets of hazardous chemical substances.

　Advances in gene recombination technology have made it comparatively easy to introduce or remove genes on the individual level. We have used mice from which we removed the gene for a metal-binding protein, metallothionein, to ascertain that the protein is involved in mitigating the toxicity of not only mercury and cadmium, but also chemical substances that produce active oxygen. As the toxicity of dioxins is manifested due to their effects, it causes the expression of genes for drug-metabolizing enzymes and a variety of other proteins. We are pursuing research to determine the mechanism by which dioxins cause gene expression, and the involvement of these genes in dioxin toxicity.