Synapsin is a neuronal phosphoprotein that coats synaptic vesicles, binds to the cytoskeleton, and is thought to function in the regulation of neurotransmitter release. Synapsin exists as a homodimer and at the subcellular level is located at the cell junction and synapse. It interacts with CAPON and forms a ternary complex with NOS1. Synapsin is a substrate of at least four different protein kinases and it is likely that phosphorylation plays a role in its regulation in the nerve terminal. Defects in SYN1 cause X-linked recessive epilepsy associated with variable learning disabilities and behaviour disorders.