A messenger RNA that encodes two or more proteins. The messenger may later be cleaved into individual messages, each of which is translated into a single protein, or a giant polypeptide chain may be translated that is later cleaved to yield the individual proteins. Polycistronic mRNAs are common in prokaryotes. For example, the lac operon (q.v.) of E. coli generates a polycistronic mRNA. Contrast with monocistronic mRNA. See histone genes, polyprotein, retroviruses, transcription unit, trans-splicing, ubiquitin.