PIWIL2 and cancer: Various studies have also showed that piRNAs could be valuable markers for cancer metastasis, for example, piR‐4987 in lymph node metastasis (Huang et al., 2013), piR‐932 and PIWIL2 in metastasis of breast cancer (Zhang et al., 2013) and piR‐32051, piR‐39894, and piR‐43607 in clear cell renal cell carcinoma metastasis, late clinical stage and poor cancer‐specific survival (Fu et al., 2015).