Our specific objectives were (i) to identify differentially expressed piRNAs and PIWI gene transcripts (mRNAs) (hereafter referred to as PIWI genes) in breast tumor tissues relative to normal (reduction mammoplasty) breast tissues, (ii) to identify piRNAs and PIWI genes as prognostic markers (outcomes: overall survival, OS and recurrence free survival, RFS) and (iii) to identify complementary gene (mRNA) targets at the 3′ UTR for the piRNAs associated with breast cancer prognosis. This evidence concerns the gene PIWIL4 and breast cancer.