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. S1A6 (S1 calcium binding protein A6) is a member of the S1 family of proteins, and functions in prolactin secretion, and exocytosis. Chromosomal rearrangements and altered expression of S1A6 have been implicated in melanoma.