While  various  positive  control  components  lie  close  to the  gene  they  regulate additional can be situated long distances away (sometimes 10-50 kb) either upstream or downstream  of the gene shown in the figure. A long-distance  positive control series of this  type  is  called as  an  enhancer  if  the  transcription  factor(s)  which  binds  to  it enhances the rate of transcription.  An enhancer is classically 100-200 bp long and haves several sequence elements which act together to give the whole enhancer activity.  When they were first found, enhancers were viewed as a distinct class of control component in which they:

?   Over long distances can activate transcription;

?   It can be located downstream or upstream of the gene being controlled;

?   Are active in moreover orientation with respect to the gene.