In molecular biology, complexin (also known as synaphin), a eukaryotic specific protein, is a cytoplasmic neuronal protein which binds to the SNARE protein complex (SNAREpin) with a high affinity. In the presence of Ca2+, the transport vesicle protein synaptotagmin displaces complexin, allowing the SNARE protein complex to bind the transport vesicle to the presynaptic membrane.