Alternative processing of the rat calcitonin gene transcript. Calcitonin and calcitonin-gene-related peptide (CGRP) have completely different protein sequences but are encoded by the same gene. The primary transcript contains two poly(A) addition sites: one used in the thyroid and the other in the brain. In the thyroid, exon 4 is retained; in the brain, exon 4 is eliminated and exons 5 and 6 are retained. The resulting mRNAs encode different polypeptides that are further processed to yield the final hormone products: calcitonin in the thyroid and CGRP in the brain.