FIGURE 1.

Schematic of two different forms of APA. (A) Tandem UTRs are characterized by the presence of an upstream proximal polyadenylation site (PAS) and a downstream distal PAS within the same terminal exon, generating multiple mRNA isoforms that differ in their 3′ UTR length without affecting the protein sequence. The use of the distal PAS leads to a longer 3′ UTR while the use of proximal PAS generates a shorter 3′ UTR. (B) Alternative last exons (ALEs) are characterized by the presence of multiple APA sites in different last exons. This results in producing both multiple mRNA and protein isoforms. The position of these ALEs is not correlated to its transcript length.