A property common to the immune system and the nervous system is regulation by a highly complex and adaptable network of cellular interactions. Major histocompatibility complex (MHC) class I molecules, which are ligands of antigen-specific receptors on CD8 T cells and of inhibitory receptors on natural killer cells, have an important and surprising role in the control of activity-dependent neuronal plasticity in the central nervous system (CNS). While expression of MHC class I molecules in neurons has been reported, corresponding immune receptors have not been identified in the CNS. Here we show selective expression of a gene related to killer cell immunoglobulin-like receptor (KIR) genes in subregions of the mouse brain where synaptic plasticity and neurogenesis occur, including olfactory bulbs, rostral migratory stream and dentate gyrus of hippocampus. These results suggest new functions for KIR-like molecules in the CNS.