Natural killer (NK) inhibitory receptors, which recognize major histocompatability complex (MHC) proteins in humans, are known as killer Ig-like receptors (KIRs) and are encoded by a multi-gene immunoglobulin (Ig) superfamily. In a screen for genes differentially expressed in the mouse thymus, we discovered the first close rodent homologue of the NK receptor KIR family, which we named KIR- Like (Kirl). KIRL1 shares 40% amino acid identity with primate KIR family members, with the majority of the homology contained within the Ig-like ectodomains. KIRL1 is more similar to the KIRs than to any other known member of the Ig domain-containing leukocyte receptor superfamily. This highly significant homology suggests that the KIR family did not arise independently in primates, as has been previously suggested, but rather evolved from a primordial gene already present in the common rodent/primate ancestor. KIRL1 lacks the cytoplasmic protein motifs that mediate inhibition in KIRs (immunoregulatory tyrosine inhibiting motif, ITIM); KIRL1 also lacks the transmembrane activation signature (a conserved K residue involved in association with the immunoregulatory tyrosine activating motif-containing DAP12 molecule) found in some KIRs. Nevertheless, we hypothesize that Kirl1 is functional, for the following reasons: (1) Kirl1 mRNA is expressed at high levels in immature thymocytes; (2) Kirl1 is regulated during thymocyte development; (3) KIRL1 protein is detected in thymus. We also show that the mouse genome contains a closely related, transcribed gene, which we name Kirl2. Kirl2 encodes a KIR-like molecule with three Ig-like domains and also lacks tyrosine-based immunoregulatory motifs in its cytoplasmic region.