Furthermore, no study has examined the role of piRNA expression in glioma, the most common adult primary malignant brain tumor [23], despite reports that some piRNAs are expressed in the mammalian central nervous system [24] and that PIWI-family protein PIWIL1 is associated with glioma tumor growth and prognosis [25, 26]. Here, PIWIL4 is linked to central nervous system cancer.