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. Human Protein S1-A8, also known as S1 calcium-binding protein A8, Cystic fibrosis antigen, Migration inhibitory factor-related protein 8, S1A8, and CAGA, is a member of the S-1 family. S1A8 plays a role in various functions of myeloid cells by forming a heterocomplex with S1A9. S1A8 and S1A9 are known to be overexpressed in certain species of carcinomas. S1A8 plays an important role in dedifferentiation of thyroid carcinoma possibly by forming a complex with S1A9. S1A8 and S1A9 may also play a key role in inflammation-associated cancer.