Refers to the portion of a gene that encodes for a part of that gene's mRNA. A gene may comprise many exons, some of which may include only protein-coding sequence; however, an exon may also include 5' or 3' untranslated sequence. Each exon codes for a specific portion of the complete protein. In some species (including humans), a gene's exons are separated by long regions of DNA (called introns or sometimes “junk DNA”) that often have no apparent function but have been shown to encode small untranslated RNAs or regulatory information. (See also splice sites.)