What is "corepressor"

n. (context genetics English) Any substance that reacts with a genetic repressor in order to activate it and decrease gene expression

In the field of molecular biology, a corepressor is a substance that inhibits the expression of genes. In prokaryotes, corepressors are small molecules whereas in eukaryotes, corepressors are proteins. A corepressor does not directly bind to DNA, but instead indirectly regulates gene expression by binding to repressors.

A corepressor downregulates (or represses) the expression of genes by binding to and activating a repressor transcription factor. The repressor in turn binds to a gene promoter (a sequence of DNA adjacent to the regulated gene), thereby blocking transcription of that gene.