PIR and breast cancer: In addition, piRNAs can also regulate gene expression at the post-transcriptional level, as in the case of piRNA-30840 which binds to IL-4 pre-mRNA, leading to its degradation (Zhong et al., 2015), and piR-FTH1, a piRNA found in breast cancer cells, which downregulates ferritin heavy chain mRNA (Balaratnam et al., 2018).