Inducer and Co-repressor
The substrate whose addition induces synthesis of an. enzyme, (as lactose in case of the synthesis of β-galactosidase), would be called inducer. Similarly, the end product, whose addition will check the synthesis of biosynthetic enzymes (as histidine in case of histidine biosynthetic enzymes), is known as co-repressor.

Although ordinarily only a substrate acts as an inducer, but rarely there are molecules that adequately resemble the natural inducers, but are not metabolized themselves by the enzyme. An excellent example is isopropyl thiogalactoside (IPTG), which, like several other galactosides, resembles lactose and thus has the property of induction. However, IPTG is not recognized by P-galactosidase. Such inducers which induce enzyme synthesis, without getting metabolized are called gratuitous inducers. Such inducers are very useful for conducting recombinant DNA studies (see Genetic Engineering and Biotechnology 1.  Recombinant DNA and PCR (Cloning and Amplification of DNA)), since they induce enzyme synthesis, but remain in the cell in original form. This also indicates that there must be sites other than target enzymes, which will recognize the substrate and utilize it in giving directions for enzyme synthesis.