Using an unbiased approach (random germline mutagenesis in mice), we have set out to identify host genes that are indispensable for survival early in the course of infection with a large DMA virus (mouse cytomegalovirus; MCMV). In selecting lethality as the endpoint of our screen, we have assured that all targets identified will be essential for defense, without regard to the level at which they function. Approximately 1 in 33 mice born to an ENU-mutagenized sire carries a recessive mutation that is lethal in the context of MCMV infection, suggesting that a large number of genes serve non-redundant functions in host resistance to this pathogen. By screening 12,170 G3 germline mutant mice from 2,028 micropedigrees, we have so far resolved a total of 31 transmissible mutations that markedly impair the innate immune response to MCMV. 8 additional mutations that impair MCMV resistance were identified by cross-screening mice identified in other innate immunity screens, so that 39 mutations have been collected in all, and at the present rate of screening, 25 new mutations should accrue each year. A total of 9 mutations have been identified already, and we have begun to map, assess allelism, and positionally clone those that remain. Informed by parallel efforts in Drosophila (Project 2) and aided by anticipatory resequencing, candidate analysis and positional cloning will be greatly accelerated. Paralogues of mutations that cause MCMV susceptibility will be targeted in mice (Project 3), and using both genetic and biochemical approaches, the mechanisms and pathways utilized in host resistance will be deciphered. Based on comparative estimates of saturation mutagenesis in which lethality is taken as an endpoint, we believe that approximately 11% of all host resistance genes have been struck in our primary screen as performed to date. As a first approximation, offer that about 280 genes comprise the MCMV resistome. The proteins that we identify may fulfill sensing or post-sensing functions. Some may offer very specific protection against MCMV; others may offer protection against many microbes, viral and non-viral alike. Some may fulfill evolutionarily conserved functions (in Drosophila and in mice) while some may be species-specific. We propose to continue the screen, which is the first of its kind in mammalian genetics, and in terms of hit rate, one of the most productive ever described in any model organism. Our goal is to elucidate a large fraction of the proteins that create resistance to viral infection in the mammalian host, and ultimately, to understand how these proteins work.