The mRNA molecule that is produced by transcription must undergo some modifications before it leaves the nucleus to direct protein synthesis. One modification involves the removal of introns. Introns are short sections of DNA that do not contain information on the protein to be built. One or more introns may be embedded in the DNA strand. The adjacent portions of the strand that do contain information on protein structure are called exons. Whether introns serve some function or are just nonsense sequences that have accidentally become inserted is under study.