Research on the regulation of transcription in mammals initially focused on the mechanism of transcriptional acti-vation and positive control of gene regulation. In contrast, transcriptional repression and negative control of gene transcription was viewed rather as part of the prokaryotic book of biology . However, results obtained in recent years have shown convincingly that transcriptional repression mediated by repressor proteins is a common regulatory mechanism in mammals and may play a key role in many biological processes. .
