Features of piRNA target sites and corresponding targeting piRNAs. (A) Grouping the 193 predicted piRNA target sites according to the number of their corresponding targeting piRNAs. (B) The distribution of the 193 predicted piRNA target sites in mRNAs in comparison with that of all mRNA fragments in the 5′-RACE library. ***P < 0.001 (Fisher's exact test). (C) Conservation of non-redundant piRNA target sites among placental mammals. (D) Sequence logo of mRNA-targeting piRNAs and their target sites in mRNAs. The logos were generated by the WebLogo web server42 using sequences of 1 295 non-redundant piRNAs and 193 target sites in mRNAs. The blue arrow marks the mRNA cleavage site between positions 10 and 11 in piRNAs. Numbers under the target mRNA logo represent the distances from the cleavage site.