A pseudogene is a sequence which is present in the genome of a given population and is typically characterized by close similarities to one or more paralogous genes, yet is nonfunctional. This lack of function is a result of either failure of transcription or translation, or production of a protein that does not have the same functional repertoire as the protein encoded by the normal paralog gene. Initial interpretation of the sequence data from human genome indicated that 10-20% of the coding sequences were pseudogenes (Dunham et al. 1999; Venter etal. 2001). There is no exact information about the number of GPCR pseudogenes but careful estimation suggests that approximately 17% of all GPCR coding sequences in the human genome are functionally inactive. The majority of vertebrate pseudogenes are a result of retrotransposition of transcripts derived from genes that encode functional proteins. By contrast, human olfactory receptor pseudogenes have been distributed to most of the human chromosomes by duplication of genomic DNA. For more than 900 identified olfactory receptors the sequence of at least 63% is disrupted by what appears to be a random process ofpseudogene formation (Glusman etal. 2001). Consideration of how pseudogenes are formed suggests that most are unlikely to be transcribed, but pseudogene transcripts can nevertheless be identified. For example, the human 5-HT7 receptor pseudogene transcripts can be identified in tissues such as kidney and liver, whereas the functional 5-HT7 receptor transcripts cannot be detected (Olsen and Schechter 1999). The functional relevance of GPCR pseudogene transcripts remains unclear.

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