Exons of the same gene may be processed and thus expressed in more than one pattern and may be recruited for the synthesis of more than one protein. It has been suggested that the role of introns in the earliest cells during evolution was to facilitate the assembly of new genes by exon shuffling. Exon shuffling is most common in vertebrates but it has also been detected in the cytosolic glyceraldehyde-3-phosphate dehydrogenase gene of plants. The ubiquitous retrotransposons of eukaryotic cells may move exons and promoters into other genes and thus generate new composite genes with different function(s). The L1 retrotransposons may be copied into the genes. They can also mobilize the L1 flanking 3′ sequences into the genes. The combination of different exons by recombination or genetic engineering may produce enzymes with potential advantage. Protein domains bordering the encoding exons are more numerous and widely distributed in the genomes indicating that these domains were amplified and interchanged more often than other domains during evolution (Liu M et al 2005 Nucleic Acids Res 33:95).  exon,  intron,  retrotransposon,  LINE,  $13′ transduction; Patthy L 1996 Matrix Biol 15:301; Kolkman JA, Stemmer WPC 2001 Nature Biotechnol 19:423.