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Gene regulation of metabolic pathways occurs when a ( regulator, operator, structural ) gene codes for a repressor molecule which binds to the operator gene causing the structural gene to be switched off and its DNA is not transcribed. In the presence of an inducer molecule the structural gene is switched on as the inducer binds with the repressor molecule making it unable to bind with the operator gene. When the inducer is broken down, the repressor molecule is no longer blocked from combining with the operator gene and the structural gene is switched back off.