Inactive but stable components of the genome derived from mutations of ancestral genes; probably account for a significant amount of the so-called "junk DNA" that comprises most of the genome of eukaryotes.

Nonfunctional DNA sequences that are very similar to the sequences of known genes. Many seem to have arisen as functional gene duplications that subsequently underwent mutation and loss of function. Others are structurally similar to mRNA and seem to have arisen by a reverse transcription event followed by integration into the genome. Some pseudogenes are transcribed and may play a role in regulation of its homologous gene.

Genes bearing close resemblance to known genes at different loci, but rendered non-functional by additions or deletions in structure that prevent normal transcription or translation. When lacking introns and containing a poly-A segment near the downstream end (as a result of reverse copying from processed nuclear RNA into double-stranded DNA), they are called processed genes.