BAI3 (Brain angiogenesis inhibitor 3) is an adhesion GPCR that plays a role in neuronal synapse formation and maintenance. It is primarily expressed post-synaptically in the cerebral cortex and on cerebellar Purkinje cells, and it binds to the TNF-like proteins C1qL1, 2, 3, and 4. BAI3 interaction with C1qL1 is required for the formation and maintenance of excitatory synapses between climbing fibers and parallel fibers with Purkinje cells. BAI3 ligation can also reduce excitatory synaptic density in hippocampal neurons, decrease dendrite arborization, and promote axon pruning in climbing fibers. It is additionally expressed in developing muscle where it is required for myoblast fusion.