A nitrogenous base, more correctly described as 4-amino-2-oxy-Pynmidine, derived from amino acids and sugars and found in all living organisms. Cytosine is found in one of the *nucleotides
present in *DNA
and pairs specifically with the Purine base guanine. Substantial amounts of cytosine in DNA and RNA are found in the modified forms of 5-methylcytosine and 5-hydroxymethyl-cytosine. The affinity of these substances for guanine is similar to that of cytosine but the significance of their occurrence is uncertain. Recent evidence suggests that the regions of DNA with a large number of methylated cytosines are inactive. Thus gene activation may be brought about, at least in part, by selective demethylation.