ATP5S, also known as ATP synthase subunits, mitochondrial, ATP synthase-coupling factor B or ATP synthase, H+ transporting, mitochondrial F0 complex, subunits (factor B), is a 215 amino acid mitochondrial inner membrane protein that belongs to the ATP synthase subunits family. Involved in regulation of mitochondrial membrane ATP synthase, ATP5S is necessary for H+ conduction of ATP synthase. The ATP5S gene encodes subunits, also known as factor B, of the proton channel. This subunit is necessary for energy transduction in ATP synthase complexes. The ATP5S gene is conserved in chimpanzee, canine, bovine, mouse, rat, chicken, zebrafish and mosquito, and maps to human chromosome 14q21.3.