PROJECT SUMMARY During brain development, the assembly of functional neural circuits necessitates sophisticated regulation of neural arborization patterns. This mechanism is achieved by a process known as neural self-avoidance, which allows projections from the same neuron to recognize and avoid self. In mammals, the regulation of neural self- avoidance relies on the expression of distinct repertoires of clustered Protocadherin (Pcdh) cell-surface protein isoforms in individual neurons. Neural-type-specific Pcdh expression is provided by a remarkable and complex mechanism of promoter selection, wherein individual neurons can choose which and how many of the 120 Pcdh isoforms (60 on each of the two parental chromosomes) to express and thereby acquire specific instructions for their connectivity patterns. For instance, convergence of olfactory sensory neuron (OSN) projections requires random expression of distinct Pcdh isoforms in individual cells, while tiling of neural arbors of serotonergic neurons (5-HTs) requires expression of the same isoform. Despite the fundamental role that Pcdh genes play in sculpting neural circuits and their implication in several devastating neurological and neuropsychiatric disorders, the molecular strategy by which individual neurons express unique combinations of Pcdh genes remains a mystery. Here we propose to investigate the mechanism of Pcdh gene choice in vivo using mouse olfactory sensory neurons and their progenitor cells by the cohesin protein complex and its regulator WAPL. The proposed studies rest on published and unpublished preliminary data that implicate cohesin in shaping the three- dimensional architecture of the Pcdh locus and its expression regulation. Our findings will not only illuminate the mechanisms of circuit assembly for olfactory sensory neurons, but they will also provide an unprecedented view of neural patterning across scales, linking cell-type-specific regulation of cohesin to neural wiring during brain development. Finally, we anticipate that our findings will also generate new hypotheses for how chromosome architecture governs gene expression more broadly.