PIWIL4 and breast cancer: Although Piwi-like (Piwil) genes including Piwil1, Piwil2, Piwil3, and Piwil4 have been detected in various types of cancer tumors, such as renal cell carcinoma (21) and breast cancer (22), overexpression of Piwil1 was the most frequently reported in cancers including lung cancer (23), gastric cancer (24), renal cancer (25) and colorectal cancer (26), in which high levels of Piwil1 showed significant correlation with short survival and/or poor prognosis in the patients with cancers (23–26).