Jacob and Monod received their share of the Nobel Prize in 1965 for their discoveries concerning the operon and viral synthesis. The first operon they described was the lac operon in E. coli. Their operon theory suggested that in all cases, genes within an operon are negatively controlled by a repressor acting at a single operator located before the first gene. Later, it was discovered that genes could be positively regulated, and also regulated at steps that follow transcription initiation. Therefore, no generalized regulatory mechanism is possible because different operons have different mechanisms. Today, an operon is defined as a functioning unit of DNA containing a cluster of genes under the control of a single promotor, transcribed together into a single mRNA strand.