Introns are DNA sequences within a gene that do not code for a protein. They are transcribed into eukaryotic pre-mRNA, but removed by splicing in the formation of a mature mRNA. In bacteria, they do not appear at all in mRNA but do in tRNA and rRNA. The term 'intron' is derived from the term intragenic region. They may have previously been protein-coding regions but due to the accumulation of mutations, perhaps from transposon activity, they became inactive. Although for a long time classed as 'junk DNA', recent evidence suggests that introns play an important role in gene regulation.

See also: exon