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{{Main|Pseudogene}}
Pseudogenes are mostly former genes that have become non-functional due to mutation, but the term also refers to inactive DNA sequences that are derived from RNAs produced by functional genes ([[Pseudogene|processed pseudogenes]]). Pseudogenes are only a small fraction of noncoding DNA in prokaryotic genomes because they are eliminated by negative selection. In some eukaryotes, however, pseudogenes can accumulate because selection is not powerful enough to eliminate them (see [[Nearly neutral theory of molecular evolution]]).
The human genome contains about 15,000 pseudogenes derived from protein-coding genes and an unknown number derived from noncoding genes.<ref>{{ cite web | url = https://useast.ensembl.org/Homo_sapiens/Info/Annotation | title = Ensemble Human reference genome GRCh38.p13}}</ref> They may cover a substantial fraction of the genome (~5%) since many of them contain former intron sequences.
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