An<unk>  is a functioning unit of key nucleotide sequences including an operator, a common promoter, and one or more structural genes, which is controlled as a unit to produce messenger RNA (mRNA), in the process of protein transcription.<unk> s occur primarily in prokaryotes but also in some eukaryotes, including nematodes. Although it may not be located in the<unk>  gene, a "Regulator" gene is present which codes for the production of a repressor or corepressor protein. The location and condition of the regulator, promoter, operator and structural DNA sequences can determine the effects of common mutations.
The first<unk>  to be described was the lac-operon in Escherichia coli, by F. Jacob, D. Perrin, C. Sanchez and J. Monod in the "Comptes rendus hebdomadaires des séances de l'Académie des sciences" in 1960.<unk> s are related to regulons and stimulons. Whereas<unk> s contain a set of genes regulated by the same operator, regulons contain a set of genes under regulation by a single regulatory protein, and stimulons contain a set of genes under regulation by a single cell stimulus.
Operon
Bleb 
Sex organ
Neck
Epidural
Head 
Skull
Tissue 