Synaptophysin is a synaptic vesicle glycoprotein with four transmembrane domains weighing 38kDa (1). It is present in neuroendocrine cells and in virtually all neurons in the brain and spinal cord that participate in synaptic transmission. It acts as a marker for the presynaptic vesicle, and functions in synaptic vesicle exocytosis, including cholesterol-dependent vesicle biogenesis, regulation of SNARE assembly via interaction with VAMP/synaptobrevin, formation of the fusion pore initiating neurotransmitter release, and activation of vesicle endocytosis (2, 3). This gene has also been implicated in X linked mental retardation (4).