S1 proteinis a family of low molecular weight protein found in vertebrates characterized by twoEF-hand calcium-binding motifs. There are at least 21 different S1 proteins, and the name is derived from the fact that the protein is1%soluble in ammonium sulfateat neutralpH. Most S1 proteins are disulfide-linked homodimer, and is normally present in cells derived from theneural crest, chondrocytes, macrophages, dendritic cells, etc. S1 proteins have been implicated in a variety of intracellular and extracellular functions. They are involved in regulation of protein phosphorylation, transcription factors, the dynamics of cytoskeleton constituents, enzyme activities, cell growth and differentiation, and the inflammatory response. Protein S1-A13, also known as S1 calcium-binding protein A13, is a member of theS-1 family. It contains twoEF-hand domains. S1A13 binds two calcium ions per subunit and one copper ion. Binding of one copper ion does not interfere with calcium binding. S1A13 is required for the copper-dependent stress-induced export of IL1A and FGF1. The calcium-free protein binds to lipid vesicles containing phosphatidylserine, but not to vesicles containing phosphatidylcholine. S1A13 plays a role in the export of proteins that lack a signal peptide and are secreted by an alternative pathway.