Synaptotagmins, such as SYT5, are a family of type III membrane proteins characterized by cytoplasmic repeats related to protein kinase C (see MIM 176960) regulatory (C2) domains, which are thought to bind calcium. Synaptotagmins may act both as negative regulators of vesicle fusion, allowing fusion in the presence of calcium, and as calcium receptors or sensor molecules (summary by Hudson and Birnbaum, 1995 [PubMed 7597049]).[supplied by OMIM, Feb 2011].