This phenomenon makes possible the prediction of the conformation of many polypeptides whose structures could not be deduced by other means. Once the structure of a module in isolation has been determined, for example by NMR spectroscopy, then the structure of homologous modules can be confidently predicted. Many mosaic proteins are constituents of the extracellular matrix or are membrane proteins, whose structures are difficult to determine by crystallographic methods. The segmental nature of these proteins indicates that the different modules have had different origins during the evolution of the genome. It appears that mosaic proteins are the result of the duplication of exons and their shuffling between different genes.
Mosaic proteins are particularly abundant in vertebrates.
In prokaryotes, gene fusion must be precise in order to preserve the reading frame of the nucleic acid. In some bacteria the enzymes involved in trp synthesis are all encoded in different genes, whereas E. coli has two bifunctional polypeptides, each the result of the fusion of two genes. A simple example of gene duplication occurs in the ferredoxins, where the two halves of the chain have a sequence consensus and a similar conformation. Note that the C-terminal regions of each half of the sequence would not be expected to show any homology, as the former forms the loop between the two halves. The tertiary structure of the F3 type module, first found in fibronectin, is shown below.