Exons are the regions of an RNA transcript, or the DNA encoding it, that remain in the mature RNA after introns have been removed by RNA splicing. In a mature mRNA, the exons can include both the protein-coding sequence, and the 5’ and 3’ untranslated regions. Exons occupy a very small portion of the whole genome. For instance, only 1.1% of the human genome is spanned by exons, whereas 24% is introns and 75% is intergenic DNA.