SV2 proteins are abundant synaptic vesicle proteins expressed in two major (SV2A and SV2B) and one minor isoform (SV2C) that resemble transporter proteins. SV2B knockout mice are phenotypically normal while SV2A- and SV2A/SV2B double knockout mice exhibit severe seizures and die postnatally. Without SV2 proteins, presynaptic Ca²⁺ accumulation during consecutive action potentials causes abnormal increases in neurotransmitter release that destabilize synaptic circuits and induce epilepsy [PMID:10624962].