The α-calcitonin gene encodes a small family of peptides: Calcitonin, katacalcin, and calcitonin generelated peptide (CGRP). Calcitonin and katacalcin are produced from one precursor and CGRP from another. Calcitonin and katacalcin come mainly from the thyroid, while CGRP is present in both the thyroid and the central nervous system. Calcitonin is concerned with skeletal integrity, while the function of katacalcin, if any, is unknown. The secretion of calcitonin is, in part, estrogen dependent, and it appears likely that a postmenopausal decline in calcitonin secretion is a factor in the development of postmenopausal osteoporosis. It is possible that calcitonin may prove useful in the prevention and perhaps the treatment of this condition. CGRP, conversely, is one of the most potent vasodilators known and probably plays an important physiologic role in the control of vessel tone and blood flow. CGRP may also have a role as neurotransmitter or neuromodulator.