Furthermore, PIWIL1 was found to be a poor prognostic factor in several tumors, including gastric cancer (Wang et al., 2012; Gao et al., 2018), soft-tissue sarcoma (Taubert et al., 2007), esophageal squamous cell carcinoma (He et al., 2009), colon cancer (Sun et al., 2017), glioma (Sun et al., 2011), hepatocellular carcinoma (Jiang et al., 2011; Zhao et al., 2012), breast cancer (Cao et al., 2016), bladder cancer (Eckstein et al., 2018), and renal cell carcinoma (Stöhr et al., 2019). This evidence concerns the gene PIWIL1 and breast cancer.