S100A13 is a member of the S100 family of proteins containing 2 EF-hand calcium-binding motifs. It is widely expressed in various types of tissues with a high expression level in thyroid gland. In smooth muscle cells, this protein co-expresses with other family members in the nucleus and in stress fibers, suggesting diverse functions in signal transduction. It plays a role in the export of proteins that lack a signal peptide and are secreted by an alternative pathway. It binds two calcium ions per subunit and one copper ion. Binding of one copper ion does not interfere with calcium binding. It 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.