PIWIL4 and cancer: Introducing external piR-FTH1 reduced Fth1 mRNA via HIWI2 and HILI, increasing TNBC cells’ doxorubicin sensitivity by 20-fold.599 In summary, piRNAs have been shown to regulate the expression of tumorigenic genes and cancer-suppressing genes at the transcriptional or post-transcriptional level by interacting with PIWI, thereby influencing the development of chemoresistance in various cancers.