Combating parasitic DNA by methylation DNA methylation plays an important role in repressing the expression of “parasitic” DNAs, such as transposable elements, which have invaded our genomes. Mammals have three DNA methyltransferase enzymes. Barau et al. discovered a fourth DNA methyltransferase enzyme in mice. The enzyme DNMT3C is a duplication of DNMT3B and is found in male germ cells. There it targets evolutionarily young transposons, of which there is a heavy burden in the mouse genome. DNMT3C methylates and silences the young transposons, preserving male fertility. Science , this issue p. 909