The control of gene expression is a complex process and most studies on the regulation of the transcription were focus on the mechanisms of transcription activation. However,transcription repression is also an important factor in the regulation of gene expression.Recent studies have found that transcription of certain genes can be downregulated in two ways.First,nucleosome may pose an obstacle to form transcription complex. Second,a set of repressive molecules inhibit transcription in a gene-specific manner. These repressors are fallen into two classes: passive repressors decrease the activity of one or more positive transcription factors by competing for DNA binding sites or by reducing DNA binding activity of the positive factors. Active repressors，on the other hand，have intrinsic repressing activity and directly inhibit transcription initiation.