Pseudogenes.

Pseudogenes are DNA sequences that bear significant homology to functional genes, yet they lack promoter sequences for their transcription or contain other mutations that preclude formation of a functional product. They appear to be a common feature of many eukaryotic genomes. Pseudogenes were first described among the 5S RNA genes of Xenopus; subsequently, they have been found for a wide range of genes, including globins, snRNAs, immunoglobulins, tubulins, and metallothionein. Some pseudogenes, like those of the beta-globins, lie within the gene cluster of their functional counterparts; they may simply have arisen by accumulation of mutations in a duplicated nonselected gene. Other pseudogenes appear to have arisen by a very different process. Characteristically, they are dispersed in the genome, lack introns, and have oligoA tracts at their 3' ends. Their structure suggests an origin from mRNAs through reverse transcription and integration into the genome. Pseudogenes have no known function. They may represent dead-end byproducts of normal cellular and evolutionary processes, yet they are also potential starting points from which new genes might evolve.