Calcitonin (salmon) is an hypocalcemic hormone produced by the parafollicular C cells of the thyroid or by the ultimobranchial bodies of nonmammalian vertebrates. Calcitonin decreases blood calcium and phosphate due to inhibition of resorption by osteoblasts and osteocytes. Calcitonin contains a single disulfide bond, which causes the amino terminus to assume the shape of a ring. Alternative splicing of the calcitonin pre-mRNA can yield a mRNA encoding calcitonin gene-related peptide; that peptide appears to function in the nervous and vascular systems. The calcitonin receptor has been cloned and shown to be a member of the seven-transmembrane, G protein-coupled receptor family.