CD274 and central nervous system cancer: CGGA database analysis: negative correlation between PD-L1 expression and the OS of glioma patients (P < 0.001) or GBM patients (P = 0.0253); TCGA database analysis: negative correlation between PD-L1 expression and the OS of glioma patients (P < 0.001) or GBM patients (P = 0.043)