Human diseases are the focus within each of the three major research areas: the biology of the immune system, metabolism, and neurobehavioral function. The mouse phenotypic screens employed in the Center for the Genetics of Host Defense identify genes that support correct functioning of the immune system, metabolism, and the nervous system. These genes (which may be identical to the human version), or similar human relatives, protect humans from a variety of diseases including immune deficiency, autoimmunity, inflammatory bowel disease, and diabetes. In this section, the diseases, and phenotypic screens used to study them, are explained briefly. Also described are select mutations that cause or modulate one of the diseases under surveillance; the affected genes are necessary to prevent disease.