The exons are the regions within a gene left behind in RNA processing after the introns are removed that are transcribed to the final messenger mRNA. Exon shuffling describes a hypothetical process in evolution whereby the exons within a gene or between two genes become mixed. Because many exons encode discrete protein domains (structural or functional units), it is hypothesized that new proteins arose in evolution by the shuffling of exons. Within this hypothesis, the evolution of new genes has often proceeded by the recombination or exclusion of exons. Exon shuffling is a probable evolutionary mechanism for the evolutionary appearance of functionally related enzymes. As a further example, the LDL receptor is an excellent example of a mosaic protein whose gene appears to have been assembled by the evolutionary process of exon shuffling.