In a paper recently published online in advance in Cell, a team led by investigators at the Fox Chase Cancer Center in Philadelphia discuss "how unmethylated regions in mammalian genomes are protected from de novo methylation and whether an active demethylating activity is involved," and show that, in mice, "either knockout or catalytic inactivation of the DNA repair enzyme thymine DNA glycosylase leads to embryonic lethality." Overall, team reports that thymine DNA glycosylase, or TDG, "keeps C