Neurotransmitter-containing synaptic vesicles (SVs) form tight clusters at synapses. These clusters act as a reservoir from which SVs are drawn for exocytosis during sustained activity. Several components associated with SVs that are likely to help form such clusters have been reported, including synapsin. Synapsin can form a distinct liquid phase in an aqueous environment. Synaptins (there are 3 variants present in humans and mice) seem to be functionally redundant, as strong phenotipical changes in the clustering of synaptic vesicules could only be observed in mice wherein all three SYN variants were knocked out. Synapsin 2 most probably performs a similar role to Synapsin 1 (PMID:29976799).