Non-coding DNA

thumb|right|300px|Human chromosomes (grey) capped by telomeres (white)

An intron is any nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word intron is derived from the term intragenic region, i.e., a region inside a gene. The term intron refers to both the DNA sequence within a gene and the corresponding RNA sequence in RNA transcripts. The non-intron sequences that become joined by this RNA processing to form the mature RNA are called exons.

semiparasitic DNA sequence, a major fraction of eukaryotic genomes

DNA not coding for protein. Often translated to RNA and critical in regulating other genes

alt=Drawing of a gene showing kinds of defects (missing promoter, start codon or introns, premature stop codon, frameshift mutation, partial deletion).|540x540px|thumb Pseudogenes are nonfunctional segments of DNA that resemble functional genes. Pseudogenes can be formed from both protein-coding genes and non-coding genes. In the case of protein-coding genes, most pseudogenes arise as superfluous copies of functional genes, either directly by gene duplication or indirectly by reverse transcription of an mRNA transcript. Pseudogenes are usually identified when genome sequence analysis finds gen

thumb|right|440px|Simplified representation of the life cycle of a retrotransposon

inherited retrovirus encoded in an organism's genome

type of regulatory sequence

type of nucleic acid sequence

An outron is a nucleotide sequence at the 5' end of the primary transcript of a gene that is removed by a special form of RNA splicing during maturation of the final RNA product. Whereas intron sequences are located inside the gene, outron sequences lie outside the gene.