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. –). A transcript map of the region identified 19 genes, including members of the killer cell lectin-like receptor family a (Klra, formerly Ly49; refs. –), 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. ), derived from Cmv1^r C57BL/6 (B6, resistant) and Cmv1^s 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).