There are three common types of alternative polyadenylation: (A) skipped-exon associated 3′ UTRs, as exemplified by the Calcitonin/CGRP gene locus, (B) composite exons associated 3′ UTRs, similar to the Immunoglobulin μ heavy chain, and (C) single terminal exons with two or more polyA sites (often called tandem UTRs), as illustrated by Dihydrofolate reductase and its seven mapped polyA sites.