In the Drosophila ovary, the I-element (a NLR retrotransposon) and gypsy are both controlled by piRNAs (small RNAs associated with Piwi-like proteins); but there are important variations on this common theme, according to the cell lineage where it operates: the germline (for the I-element) versus the ovarian somatic cells (for gypsy). We are taking advantage of these differences to study the piRNA biogenesis, the epigenetic effects of the maternal transmission of piRNAs and the mechanism of piRNA-mediated regulation.