Exon shuffling is a molecular mechanism for the formation of new genes. It is a process through which two or more exons from different genes can be brought together ectopically, or the same exon can be duplicated, to create a new exon-intron structure. There are different mechanisms through which exon shuffling occurs: transposon mediated exon shuffling, crossover during sexual recombination of parental genomes and illegitimate recombination. Exon shuffling follows certain splice frame rules. Introns can interrupt the reading frame of a gene by inserting a sequence between two consecutive codons (phase 0 introns), between the first and second nucleotide of a codon (phase 1 introns), or between the second and third nucleotide of a codon (phase 2 introns). Additionally exons can be classified into nine different groups based on the phase of the flanking introns (symmetrical: 0-0, 1-1, 2-2 and asymmetrical: 0–1, 0–2, 1–0, 1–2, etc.) Symmetric exons are the only ones that can be inserted into introns, undergo duplication, or be deleted without changing the reading frame. History Exon shuffling was first introduced in 1978 when Walter Gilbert discovered that the existence of introns could play a major role in the evolution of proteins. It was noted that recombination within introns could help assort exons independently and that repetitive segments in the middle of introns could create hotspots for recombination to shuffle the exonic sequences. However, the presence of these introns in eukaryotes and absence in prokaryotes created a debate about the time in which these introns appeared. Two theories arose: the "introns early" theory and the "introns late" theory. Supporters of the "introns early theory" believed that introns and RNA splicing were the relics of the RNA world and therefore both prokaryotes and eukaryotes had introns in the beginning. However, prokaryotes eliminated their introns in order to obtain a higher efficiency, while eukaryotes retained the introns and the genetic plasticity of the ancestors. On the other hand, supporters of the "introns late" theory believe that prokaryotic genes resemble the ancestral genes and introns were inserted later in the genes of eukaryotes. What is clear now is that the eukaryotic exon-intron structure is not static, introns are continually inserted and removed from genes and the evolution of introns evolves parallel to exon shuffling. In order for exon shuffling to start to play a major role in protein evolution the appearance of spliceosomal introns had to take place. This was