In molecular biology and genetics, transcription coregulators are proteins that interact with transcription factors to either activate or repress the transcription of specific genes.  Transcription coregulators that activate gene transcription are referred to as coactivators while those that repress are known as corepressors.  The mechanism of action of transcription coregulators is to modify chromatin structure and thereby make the associated DNA more or less accessible to transcription. In humans several dozen to several hundred coregulators are known, depending on the level of confidence with which the characterisation of a protein as a coregulator can be made. One class of transcription coregulators modifies chromatin structure through covalent modification of histones.  A second ATP dependent class modifies the conformation of chromatin.