Nurse cell-derived small RNAs define paternal epigenetic inheritance in Arabidopsis Genomes of germ cells present an existential vulnerability to organisms because germ cell mutations will propagate to future generations. Transposable elements are one source of such mutations. In the small flowering plant Arabidopsis, Long et al. found that genome methylation in the male germline is directed by small interfering RNAs (siRNAs) imperfectly transcribed from transposons (see the Perspective by Mosher). These germline siRNAs silence germline transposons and establish inherited methylation patterns in sperm, thus maintaining the integrity of the plant genome across generations. 373 6550 American Association for the Advancement of Science 1