Inducible operon is an operon which remains switched off normally but becomes operational in the presence of an inducer (lactose in case of lac operon). The inducible operon generally functions in catabolic pathways. In the presence of an inducer, i.e,, lactose (actually allolactose or metabolite of lactose), the repressor has a higher affinity for the inducer than for the operator gene. When lactose is added, a few lactose molecules are carried into the cell by the enzyme lactose permease as small amount of this enzyme is present in the cell even when the operon is not working. These few lactose molecules are converted into allolactose molecules which act as an inducer and bind to the repressor (a product of regulator gene). The repressor-inducer complex fails to join with the operator gene, thus it is turned on.