The pattern of DNA methylation plays an important role in regulating differ
ent genome functions. To test the hypothesis that DNA methylation is a reve
rsible biochemical process, we purified a DNA demethylase from human cells
that catalyzes the cleavage of a methyl residue from 5-methyl cytosine and
its release as methanol. We show that-similar to DNA methyltransferase, DNA
demethylase shows CpG dinucleotide specificity, can demethylate mdCpdG sit
es in different sequence contexts, and demethylates both fully methylated a
nd hemimethylated DNA. Thus, contrary to the commonly accepted model, DNA m
ethylation is a reversible signal, similar to other physiological biochemic
al modifications.