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In genetics and biochemistry, a repressor gene inhibits the activity of an operator gene. Removal of repression, such as of an operon so that gene transcription occurs or is enhanced, with the net result frequently being elevation of the level of a specific enzyme. This effect is called derepression.

DALA synthase[edit]

One example is the effect of drugs on the activity of ALA synthase, an enzyme involved in heme-synthesis. When inducing for example phenobarbital, griseofulvin or hydantoins, the activity of ALA synthase will rise. The cause is a production of cytochrome P450-production, induced by the drugs, consuming heme. When the amount of heme in the liver decreases, it no longer act as a repressor for its synthesis. On the contrary, ALA synthase increases which leads to an increase in heme-production.