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. Other scaffolding proteins, for example SH3 domain-containing proteins, could coassemble into this condensate but were not necessary for its formation. Importantly, the synapsin phase could capture small lipid vesicles in vitro. The synapsin phase rapidly disassembled upon phosphorylation by calcium/calmodulin-dependent protein kinase II (CaMKII), this calcium-dependent phosphorylation probably serves the release of vesicles from clusters during sustained activity of the nerve terminal. In synapsin KO mice the number and packing of SVs were significantly lower and this decrease is selective for SVs away from active zones. Thus synapsin can form a separate liquid biomolecular condensate either alone or together with binding partners for its IDR, with lipid vesicles, or with both (PMID:29976799).