Retrotransposition and exonization of an Alu element in the first intron of TIF-IA gene in locus 21. (A) All mammalians genomes (except for opossum) contain a LINE element (L2) in intron 1 of TIF-IA gene. During primate evolution, an Alu element was inserted into L2. The L2 and Alu elements accumulated mutations leading to exonization (L2-AEx), in which a 3'ss and two alternative 5'ss are recognized by the splicing machinery. Three alternatively spliced isoforms are generated following this exonization: (i) a skipping isoform with no L2-AEx; (ii) selection of a distal 5'ss (termed 5'ss-a), which generates a 180-nt L2-AEx; and (iii) selection of a proximal 5'ss (termed 5'ss-b), leading to exonization of a 370-nt L2-AEx (left to right, respectively). (B) Multiple alignment of TIF-IA splice sites and flanking regions in all three loci. Splice sites are marked with arrows on the top. Exonic and intronic sequences are in uppercase and lower case, respectively. Mutations relative to the original gene (locus 15) are highlighted in pink. 5'ss-a is the distal splice-site and 5'ss-b is the proximal one.