isopropyl thiogalactoside (IPTG)

Recall that lactose utilization in E. coli  is controlled by three contiguous genes:

lacZ  (b-galactosidase).
lacY  (b-galactoside permease).
lacA  (thiogalactoside transacetylase).

In the presence of an inducer, all three proteins accumulate simultaneously, but to different levels.

Lactose itself leads to induction of the lactose operon,
but the true intracellular inducer is allolactose, Galb(16)Glc, a minor product of b-galactosidase action.

In the laboratory, one usually uses a synthetic inducer such as isopropyl thiogalactoside (IPTG), which induces the lactose operon but is not cleaved by b-galactosidase.

Hence, its concentration does not change during an experiment.