Advances in prevention and treatment of autoimmune diseases, allergy, transplant rejection, and infection depend upon identifying key regulatory genes and biochemical pathways in the immune system that can be used to enhance diagnostic specificity and as targets for new drugs or vaccines. This project will take advantage of the recently completed sequencing of the human and mouse genome and a new approach of genome-wide mutagenesis to produce new animal models of immunological disease, to identify key regulatory genes, and to elucidate cellular and molecular pathways underpinning immune regulation. A C57BL/6 transgenic mouse model will be used as the starting point for genome-wide mutagenesis, using the chemical supermutagen ENU to alter the function of tens of thousands of genes in the mammalian genome in a highly parallel method. A consortium of investigators will develop and validate screening tests to identify mice with altered immune regulation, and then define the mutant gene, molecular pathways, and cellular processes revealed by each new mouse strain. The consortium will establish, characterize and disseminate many new mouse strains with mutations that reveal critical genes and pathways for immunity versus tolerance decisions: regulating development, inactivation and activation of different subsets of T cells, B cells, and NK cells; counterbalancing signals from activating and inhibitory receptors; guiding leukocyte migration and microenvironmental interactions; and establishing response differences in isotype-switched memory cells. The results of this work will identify promising targets and strategies for improving the success of therapeutic tolerance in autoimmunity, allergy, and transplantation, and for enhancing immunity to infectious diseases.