A transcript which has undergone the necessary modifications, if any, for its function. In eukaryotes this includes, for example, processing of introns, cleavage, base modification, and modifications to the 5' and/or the 3' ends, other than addition of bases. In bacteria functional mRNAs are usually not modified. [ http://www.sequenceontology.org/browser/current_svn/term/SO:ke ]