An exon is a nucleic acid sequence that's represented in the mature form of an RNA molecule after a) portions of a precursor RNA, introns, have been removed by cis-splicing or b) two or more precursor RNA molecules have been ligated by trans-splicing. The mature RNA molecule can be a messenger RNA or a functional form of a non-coding RNA such as rRNA or tRNA. Depending on the context, exon can refer to the sequence in the DNA or its RNA transcript. History The term exon was coined by American b… (More on Exons)