Self avoidance is essential for the proper development of the nervous system. Growing dendrites and axons must first recognize and then avoid sister processes derived from the same soma in order to promote their even distribution within the environment. This process of self recognition was recently demonstrated to be mediated via the protocadherin family. Because of this relatively new discovery, comparatively little is known regarding how protocadherins act to effect self recognition. In Drosophila, self recognition is mediated primarily by DSCAM1. The goal of this proposal is to draw upon extensive prior studies of DSCAM1 in Drosophila to inform protocadherin function in vertebrates. A fundamental principle in Drosophila is the finding that individual neurons express different complements of DSCAM1 isoforms. This endows each neuron with a unique molecular signature that enables the neuron to recognize self from non-self. However, determining whether this same basic principle also holds true in vertebrates is practically difficult. The sheer number of neurons, coupled with a lack of molecular markers that can label neuronal subtypes, has made it hard to test if each neuron expresses a unique protocadherin signature. To address this question, this proposal will utilize powerful genetic tools in the olfactory system that can label neurons expressing a common odorant receptor. This allows the isolation and examination of protocadherin expression in just 1/1000th of all olfactory neurons. Single cell RNA analysis will be used to determine whether these highly related neurons do indeed express different combinations of protocadherins. This approach will also determine how many protocadherins are expressed by a given neuron, and whether or not the expression appears random. The impact of these combinations on self recognition will then be tested using canonical cell adhesion assays. This proposal will therefore provide one of the highest resolution tests of self recognition to date in vertebrates, and will compare and contrast principles of self recognition across phyla. Understanding how protocadherin expression patterns arise during development has broader implications for human health. Protocadherins are associated with or are the causative mutation behind several neurological disorders. These studies will provide a foundation for understanding how loss of even a single protocadherin can lead to disease.