Offspring affected by sperm small RNAs: Paternal dietary conditions in mammals influence the metabolic phenotypes of offspring. Although prior work suggests the involvement of epigenetic pathways, the mechanisms remains unclear. Two studies now show that altered paternal diet affects the level of small RNAs in mouse sperm. Chenet al.injected sperm transfer RNA (tRNA) fragments from males that had been kept on a high-fat diet into normal oocytes. The progeny displayed metabolic disorders and concomitant alteration of genes in metabolic pathways. Sharmaet al.observed the biogenesis and function of small tRNA-derived fragments during sperm maturation. Further understanding of the mechanisms by which progeny are affected by parental exposure may affect human diseases such as diet-induced metabolic disorders.Science, this issue p.397, p.391