Calcitonin is a hormone known to participate in calcium and phosphorus metabolism. In mammals, the major source of calcitonin is from the parafollicular or C cells in the thyroid gland. Calcitonin is a 32 amino acid peptide cleaved from a larger prohormone. It 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.