Two examples of piRNA clusters. (A) Bidirectional; (B) Unidirectional. The genomic location of the locus, piRNA mapping density, and GENCODE gene annotation are shown. piRNAs mapped to the sense and antisense strand of the reference genome are shown in brown and blue, respectively. The mapping density plot is generated by UCSC genome browser Custom Tracks tool. For gene annotation, exonic regions are shown as solid boxes, while non-exonic regions are shown as thin lines, with arrows indicating the direction of the gene.