DDX4 and testicular germ cell tumor: Downregulation of piRNA pathway genes in cancer may be uncommon but have been reported in testicular germ cell tumor (PIWIL1, PIWIL2, PIWIL4 and DDX4) and renal cell carcinoma (PIWIL1, PIWIL2 and PIWIL4) and now, also in our expression study for PIWIL2, PIWIL4 and TDRD1 [63,64].