Potential models for selecting transcripts for piRNA processing. (a) A unique chromatin state of piRNA-producing loci can guide channeling of RNA transcribed from these regions to the piRNA processing machinery. Bidirectional piRNA clusters are enriched in the HP1 homolog Rhino. Rhino colocalizes with UAP56, which binds long piRNA precursors and shuttles them from the nucleus to cytoplasmic nuage granules. These interactions could facilitate targeted export of cluster transcripts directly to the processing machinery. (b) Trans-generationally inherited piRNAs can guide selection of the transcripts processed to piRNA. Maternally transmitted piRNAs loaded into Aub and Ago3 complexes can initiate processing of complementary transcripts through the Ping-Pong cycle. Alternatively, inherited piRNAs bound to the nuclear Piwi protein could identify nascent transcripts and induce deposition of chromatin marks such as H3K9me3 that lead to processing of transcripts into piRNAs using the pathway shown in (a).