Another mechanism involving the structure of the 3′-UTR is called alternative polyadenylation (APA), which results in mRNA isoforms that differ only in their 3′-UTRs. This mechanism is especially useful for complex organisms as it provides a means of expressing the same protein but in varying amounts and locations. It is utilized by about half of human genes. APA can result from the presence of multiple polyadenylation sites or mutually exclusive terminal exons. Since it can affect the presence of protein and miRNA binding sites, APA can cause differential expression of mRNA transcripts by influencing their stability, export to the cytoplasm, and translation efficiency.