An intein is a segment of a protein that's able to excise itself and rejoin the remaining portions (the exteins) with a peptide bond. Inteins have also been called "protein introns". Most reported inteins also contain an endonuclease domain that plays a role in intein propagation. In fact, many genes have unrelated intein-coding segments inserted at different positions. For these and other reasons, inteins (or more properly, the gene segments coding for inteins) are sometimes called selfish gene… (More on Intein)