Cytomegalovirus is the leading cause of congenital viral disease and the most important opportunistic infection in immunocompromised patients. We have used a mouse experimental infection model (MCMV) to study the genetic parameters of host/virus interaction. Susceptibility to infection with MCMV is controlled by Cmv1, a chromosome 6 locus that regulates natural killer (NK) cell activity against virally infected targets. Here, we use a positional cloning strategy to isolate the gene mutated at the Cmv1 locus. Cmv1 maps within a 0.35-cM interval defined by markers D6Ott8 and D6Ott115, which corresponds to a physical distance of 1.6 Mb (refs. 6-8). A transcript map of the region identified 19 genes, including members of the killer cell lectin-like receptor family a (Klra, formerly Ly49; refs. 9-12), which encode inhibitory or activating NK cell receptors that interact with MHC class I molecules. Klra genes have different copy numbers and genomic organization, and are highly polymorphic among inbred strains, making it difficult to distinguish between normal allelic variants and distinct Klra genes, or possible mutations associated with Cmv1. The recombinant inbred strain BXD-8/Ty (BXD-8; ref. 18), derived from Cmv1r C57BL/6 (B6, resistant) and Cmv1s DBA/2 (susceptible), is of particular interest because it is highly susceptible to MCMV infection despite having a B6 haplotype at Cmv1. We determined that MCMV susceptibility in BXD-8 is associated with the deletion of Klra8 (formerly Ly49h).