Analogous to what was found in the 5′UTR, also the presence of an intron in the 3′UTRs may influence gene expression. 3′UTRs are generally much longer than 5′UTRs, but relatively few 3′UTRs (<5%) contain introns.46 The reason could be partially explained by nonsense-mediated decay, by which transcript degradation would be typically signalled by an intron downstream of the stop codon.77 In addition, splicing signals within 3′UTRs have been suggested to have reduced maintaining selection, being the 3′UTRs better able to tolerate loss of intron integrity than other gene regions; consequently, 3′UTRs tend to be longer with fewer introns compared to 5′UTRs.78
