PIWI-interacting small non-coding RNAs (piRNAs) are genetic and epigenetic 
regulatory factors in germline cells, where they maintain genome stability, are 
involved in RNA silencing and regulate gene expression. We found that the piRNA 
biogenesis and effector pathway are present in human breast cancer (BC) cells 
and, analyzing smallRNA-Seq data generated from BC cell lines and tumor 
biopsies, we identified >100 BC piRNAs, including some very abundant and/or 
differentially expressed in mammary epithelial compared to BC cells, where this 
was influenced by estrogen or estrogen receptor β, and in cancer respect to 
normal breast tissues. A search for mRNAs targeted by the BC piRNome revealed 
that eight piRNAs showing a specific expression pattern in breast tumors target 
key cancer cell pathways. Evidence of an active piRNA pathway in BC suggests 
that these small non-coding RNAs do exert transcriptional and 
post-transcriptional gene regulatory actions also in cancer cells.