Calcitonin and calcitonin gene related peptide (CT/CGRP) are coded by a commensal gene, of which the structure and 5＇ flanking sequence make the differential expressive products from thyroid C cell or nerve cell. The regulation of alternative expression determines the growth, sexual differentiation and evolution in multicellular animals. The medullary thyroid carcinoma (MTC) or osteoporosis would arise supposing that the abnormal expressive regulation occurs.