DEOXY-5-METHYLCYTIDYLIC acid (5-MeC) is an almost universal, although minor component of DNA in plants and animals. It makes up 4–7% of the bases in plants1, no more than 1.5% of the bases in the mouse2 and even less in the human3. 5-MeC itself is not incorporated directly into DNA. Instead, specific deoxycytidylic acid residues in preformed DNA are methylated enzymatically by a highly specific DNA methylase, which transfers a methyl group from S-adenosyl methionine to specific sites in DNA4. In the mouse, as in other mammals, virtually all of the 5-MeC is in the 5-MeCpG doublet5, which is distributed non-randomly in the DNA. Mouse satellite DNA contains about 3% 5-MeC in comparison to about 1.3% in main band DNA2 amd is located at the centromeric end of nearly every chromosome6.