SV2s (Synaptic Vesicle protein 2) are integral membrane glycoproteins present in all synaptic vesicles. They have 12 transmembrane domains predicted by sequence analysis. There are three characterized isoforms, SV2A, SV2B and SV2C. SV2A is expressed ubiquitously throughout the brain. SV2B has a more restricted distribution with varying degrees of coexpression with SV2A. SV2C is more closely related to SV2A but shows a very restricted expression pattern. The highest expression levels were observed in phylogenetically old brain areas like pallidum, the midbrain and the olfactory bulb. SV2A is probably involved in the maintenance of a pool of synaptic vesicles competent for calcium-stimulated exocytosis. SV2A is a highly glycosylated synaptic vesicle protein with homology to transmembrane transporters.