Association of 3’ UTR-directed piRNAs in Piwi-protein complexes. (A) RNAs from PIWI, MILI, and MIWI IPs were probed on Northern blots with cloned 3’ UTR fragments. (B) The 3’ UTR probes are specific for piRNAs from Piwi-protein IPs. (C) Deep conservation of 3’ UTR-directed piRNAs in animals. Enrichment of piRNA reads to the 3’ UTR of KLHL7 and DDX3X genes from an IP of Xenopus Piwi proteins with the Y12 antibody, as illustrated by the gene map and the bar graph of normalized piRNA read density according to transcript feature. 5’ UTR, CDS and 3’ UTR features are represented by blue, dark grey, and red colors, respectively.