Repressor - The substance which stops or represses the expression of specific genes is called as repressor and the phenomenon is called as repression

If the presence of a specific regulatory element enhances the level of gene expression, then that is called as positive regulation and the molecule is called as activator, e.g. lac operon

If the presence of a specific regulatory molecule will reduce the gene expression, then it is called as negative regulation and the molecule is called as repressor.

Jacob and Monad in 1961 proposed a model which explains how a bacteria metabolizes lactose and tryptone, and the model is called as Jacob-Monad model. Bacteria use a positive control to metabolize lactose and negative control or regulation to metabolize tryptone.