Domain Shuffling
 
Proteins are often composed of one or more functional domains. In particular, when a certain protein possesses more than one domain, it is called a mosaic protein. Domain shuffling is a phenomenon  where the gene segments coding for functional domains are shuffled between different genes during evolution. The boundaries between different domains may or may not correspond to the intron–exon boundaries of the encoding gene. In fact, in certain cases a single domain is encoded by more than one exon, whereas an exon contains a number of functional domains. Therefore, the “exon shuffling” theory is not necessarily the same as the “domain shuffling” theory. Although there are many cases where domain shuffling must have taken place, it is not known what molecular event causes this, and this is important to determine.