(Messenger RNA). An RNA which contains sequences coding for a protein. The term mRNA is used only for a mature transcript with poly(A) tail and with all introns removed, rather than the primary transcript in the nucleus. As such, an mRNA will have a 5′ untranslated region, a coding region, a 3′ untranslated region, and (almost always) a poly(A) tail. Typically about 2% of the total cellular RNA is mRNA.