Visualization of different piRNA clusters. Every illustration of a piRNA cluster is associated with some sequence annotation which refers to the presence of repetitive sequences and genes. 4a: Cluster detected with proTRAC, not annotated at piRNABank. 4b: Cluster annotated at piRNABank, rejected by proTRAC because of its atypical topology. 4c: Upper illustration shows a cluster (not annotated by Lau et al. 2006  but detected with proTRAC) arisen from a segmental duplication of the lower cluster. 4d: Two visualizations of the same cluster detected with different methods. Upper cluster (proTRAC) includes sequence reads that map more than 5 times to the genome, lower cluster (Girard et al.  method) does not include sequence reads that map more than 5 times to the genome. Black bars: single-copy loci, red bars: multi-copy loci.