S100 family proteins are small (10-12 kDa), acidic proteins that are found exclusively in vertebrates. They belong to a highly conserved group of EF-hand calcium-binding molecules that consists of more than 25 members. They are the only EF-hand proteins that form both homodimeric and heterodimeric complexes. Although structurally related, their expression patterns and functions are usually distinct.

Human S100P is a 22 kDa, homodimeric member of the S100 family of calcium-binding proteins. The S100 family currently has more than 20 members and belongs to the EF-hand superfamily of molecules. S100P is 95 aa in length and contains short, sequential modules. There is an N-terminal alpha-helix, a unique EF-hand motif, an alpha-helix, a linker region, an alpha-helix, a classic EF-hand motif and a C-terminal alpha-helix. The EF-hand motif binds calcium, which likely alters molecular conformation. The rearranged S100P now binds ligand with the linker region. Human S100P is 47% aa identical to rat S100P.