1. An enzyme that can be detected in a growing culture of a microorganism, after the addition of a particular substance (inducer) to the culture medium, but was not detectable prior to the addition and can act on the inducer. A prototype is the ß-galactosidase of Escherichia coli, synthesized on the addition of various galactosides, regardless whether these are good substrates. 2. Any enzyme that has its rate of biosynthesis increased due to the presence of the substrate or some other molecular entity.