CCND2 and central nervous system cancer: As shown Fig. 3, FSTL5, HCN1, TMEM132D, TRHDE, KRT222, CACNA1B, CDH9, SLC22A9, GABRA1 and GABRB2 showed the strongest expression correlation with other genes in glioma tissues (Additional file 1: Table S7), while, PRKAR2B, CCND2, C1orf173, WBSCR17, STXBP5L, PRKCE, KIF3A, GRAMD1B, SLC1A6 and ADCY1 were the hub genes in the co-expression network of normal brain tissues (Fig. 4; Additional file 1: Table S8).