active repressor

n.
  1. A repressor that binds directly with an operator gene to block it and its structural genes from synthesizing enzymes; it is a homeostatic mechanism for the regulation of inducible enzyme systems.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.