1) Coding sequence of DNA present in mature messenger RNA; DNA initially transcribed to messenger RNA consists of coding sequences (exons) and non-coding sequences (introns). Introns are spliced out of the messenger RNA prior to translation, leaving only the exons to ultimately encode the amino acid product.

2) The protein-coding DNA sequence of a gene.

3) 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.