After 8 Gy irradiation, the miR-21 level increased up to 2.0 fold or more compared to the endogenous level in PTEN mutant glioma cells (2.1 fold in U87 cells, and 2.2 fold in U373 cells), while 1.4 fold or less in PTEN wild type glioma cells (1.4 fold in LN 18 cells and 0.87 fold in LN 428 cells) (Figure 1B). Here, PTEN is linked to central nervous system cancer.