The inducer for Lac operon of Escherichia coli is
lactose (actually allolactose or metabolite of Lactose). This
lac operon normally remains inactive. When lac operon
contacts with lactose, the lactose acts as an inducer and
combines with the repressor, and the repressor is detached
from operator gene. Thus RNA polymerase enzyme gets
its passage and reaches to the structural genes and starts
the transcription.