Helitron element and its downstream genomic regions, flanked by a random DNA site, serving as a "de novo" RC terminator. According to the RTM2 model the 3' terminus of another Helitron serves as an RC terminator of transposition. This occurs after a malfunction of the RC terminator. Lastly in the FDNA model portions of genes or non-coding regions can accidentally serve as templates during repair of ds DNA breaks occurring in helitrons. Even though helitrons have been proven to be a very important evolutionary tool, the specific details for their mechanisms of transposition are yet to be defined. An example of evolution by using helitrons is the diversity commonly found in maize. Helitrons in maize cause a constant change of genic and nongenic regions by using transposable elements, leading to diversity among different maize lines. Long-terminal repeat (LTR) retrotransposons Long-terminal repeat (LTR) retrotransposons are part of another mechanism through which exon shuffling takes place. They usually encode two open reading frames (ORF). The first ORF named gag is related to viral structural proteins. The second ORF named pol is a polyprotein composed of an aspartic protease (AP)which cleaves the polyprotein, an Rnase H (RH) which splits the DNR-RNA hybrid, a reverse transcriptase (RT) which produces a cDNA copy of the transposons RNA and a DDE integrase which inserts cDNA into the host's genome. Additionally LTR retrotransponsons are classified into five subfamilies: Ty1/copia, Ty3/gypsy, Bel/Pao, retroviruses and endogenous retroviruses. The LTR retrotransponsons require an RNA intermediate in their transposition cycle mechanism. Retrotransponsons synthesize a cDNA copy based on the RNA strand using a reverse transcriptase related to retroviral RT. The cDNA copy is then inserted into new genomic positions to form a retrogene. This mechanism has been proven to be important in gene evolution of rice and other grass species through exon shuffling. Transposons with Terminal inverted repeats (TIRs) DNA transposon with Terminal inverted repeats (TIRs) can also contribute to gene shuffling. In plants, some non-autonomous elements called Pack-TYPE can capture gene fragments during their mobilization. This process appears to be mediated by acquisition of genic DNA residing between neighbouring Pack-TYPE transposons and its subsequent mobilization. Illegitimate recombination Lastly, illegitimate recombination (IR) is another of the mechanisms through which exon shuffling occurs. IR is the recombination between short homologous sequences or nonhomologous sequences. There are two classes of IR: The first corresponds to errors of enzymes which cut and join DNA