Transposon Silencing by piRNAs

In animal cells, small RNA molecules, called piRNAs, defend the genome against selfish DNA elements such as transposons. In this issue, Klattenhoff et al. (2009) report that an HP1 family protein, Rhino, is required for piRNA generation and transposon silencing in Drosophila germline cells. The results provide a link between heterochromatin and piRNA-mediated genome defense.