The mechanisms that enable olfactory sensory neurons in the mouse nasal epithelium to target distinct glomeruli in the main olfactory bulb remain unresolved. In order to provide new information on this issue, we propose the following experiments: In Specific Aim 1, we will test the role that major histocompatibility class I molecules play in axon guidance. To accomplish this, we will examine gene-targeted mice that have tau-lacZ labeled olfactory sensory neurons and are deficient in the presentation of class I MHC signaling molecules on cell surfaces. In Specific Aim 2, we will use microarray gene chip technology to identify potential guidance cues that are differentially expressed in the main olfactory bulb. We will also examine the effect that nerve transection has on gene expression. These experiments should significantly further our understanding of axonal targeting in the main olfactory bulb.