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-A12, also known as S1 calcium-binding protein A12, Calcium-binding protein in amniotic fluid 1, Calgranulin-C, and S1A12, is a member of the S-11 family. Like the majority of S1 proteins, S1A12 is a dimer, with the interface between the two subunits being composed mostly of hydrophobic residues. The fold of S1A12 is similar to the other known crystal and solution structures of S1 proteins, except for the linker region between the two EF-hand motifs. S1A12 plays an important role in the inflammatory response.