In humans, DNA methylation occurs at the 5′ position of the pyrimidine ring of the cytosine residues within CpG sites to form 5-methylcytosines.  The presence of multiple methylated CpG sites in CpG islands of promoters causes stable inhibition (silencing) of genes.  Silencing of transcription of a gene may be initiated by other mechanisms, but this is often followed by methylation of CpG sites in the promoter CpG island to cause the stable silencing of the gene.