intron

An intron is a stretch inside a gene that gets cut out before the message is used — like a scene shot but left on the cutting-room floor. It is transcribed into the first RNA copy but then removed during splicing, so it never appears in the protein-coding instructions that reach the ribosome.

Introns interrupt the coding parts (the exons) of most genes in animals, plants, and other eukaryotes. Their boundaries carry signal sequences that the spliceosome reads to make precise cuts. Because introns are removed, their internal sequence is under less pressure to be perfect, though their splice sites must stay intact for the gene to work.

Far from being mere junk, introns can carry regulatory elements, host other functional RNAs, and enable alternative splicing, which lets one gene yield several proteins. Mutations that disrupt a splice site can cause an intron to be wrongly kept or an exon to be skipped, and such errors underlie a number of inherited disorders.