An exon is the portion of a gene that codes for amino acids. In the cells of plants and animals, most gene sequences are broken up by one or more DNA sequences called introns. The parts of the gene sequence that are expressed in the protein are called exons, because they are expressed, while the parts of the gene sequence that are not expressed in the protein are called introns, because they come in between or interfere with the exons. Related Theory Exons usually include both the 5' and 3' untranslated regions of mRNA, which contain start and stop codons, in addition to any protein coding sequences