Cytosine  may become methylated (addition of CH3 groups), which alters
   the binding of proteins to the DNA helix and depresses transcription.

   The  base  analog  5-azacytosine  has a nitrogen at the 5 position and
   cannot  be  methylated. When this is incorporated into DNA in place of
   Cyteosin,  the  result  is  undermethylation  and  stimulation of gene
   expression.