PIWIL4 and cancer: The four human PIWI proteins PIWIL1, PIWIL2, PIWIL3 and PIWIL4 can be included in the cancer/testis antigens (CTAs) class and their deregulation is involved in cancer cell proliferation, apoptosis and stemness [28,29,30,31,32,33,34], genomic integrity [27], invasion and metastasis [35,36,37,38].