S1 protein?is a family of low molecular weight protein found in vertebrates characterized by two?EF-hand calcium-binding motifs. There are at least 21 different S1 proteins, and the name is derived from the fact that the protein is?1%?soluble in ammonium sulfate?at neutral?pH. Most S1 proteins are disulfide-linked homodimer, and is normally present in cells derived from the?neural 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-A5, also known as Protein S-1D, S1 calcium-binding protein A5, S1A5 and S1D, is a member of the S1 family which contains two?EF-hand domains. S1A5 is also a novel member of the EF-hand superfamily of calcium-binding proteins that is poorly characterized at the protein level. It is expressed in very restricted regions of the adult brain. From birth onwards, S1A5 remained a neuronal-specific protein, only located in a subpopulation of neurons in the spiral ganglion.