Approximately the 35% of human genes contain introns in the 5′UTR.45 These 5′ introns are less common than introns within coding regions, but are, on average, longer.46 By analysing the expression profiles of genes with 5′UTR introns, Cenik and colleagues found that the most highly expressed genes reveal a strong enrichment of short 5′UTR introns with respect to long or absent 5′UTR introns.18 No relationship was found between length and expression level for genes with intermediate or long 5′ introns. Considering that expression depends on production and degradation rates of mRNAs, Cenik’s results suggest that short 5′UTR introns tend to increase transcription or stabilize mature mRNAs.
