PIR and breast cancer: Complex formed by piRNAs binding to Piwi proteins, such as piR-651/Piwil 1 in gastric cancer (24), piR-54265/Piwil 2 in colorectal cancer (37), piR-932/Piwil 2 in breast cancer (29), have been demonstrated to regulate cell proliferation, invasion and metastasis of cancer cells.