Annotation of A5EΔ4 splicing exons in REFSEQ genes. Percentages refer to fractions of A5EΔ4 splicing exons located in the 5'-UTR, coding sequence (CDS) region, or 3'-UTR. A black-colored "s" indicates the position of the stop codon relative to the REFSEQ transcript structure, whereas the red-colored version indicates the altered stop codon due to tandem donor splicing. A5EΔ4 splicing exons embedded within CDS regions are broken down into two categories, depending on the creation of a premature (PTC) or delayed termination codon (DTC). PTCs can signal mRNAs as substrates for non-sense mediated decay.