The birth of an exon through RNA editing (Lev-Maor et al. 2007). Shown is a schematic illustration of the genomic region spanning exons 7–9 of the human NARF gene (not to scale). Exons are depicted as cylinders. The Alu element that is the source of the new exon is orange; an intronic, antisense orientation Alu sequence (light blue) is 25 bp upstream of the exonized Alu. Sense and antisense Alus fold to form a double-stranded RNA (dsRNA) secondary structure, thus allowing RNA editing to take place (lower panel). RNA editing changes an AA dinucleotide into a functional AG 3′ splice site and also changes a UAG stop codon into a UGG Trp codon. Thus, RNA editing leads to the creation of a new functional exon.
