CDK4 and central nervous system cancer: In contrast, the expression of glial fibrillary acidic protein (GFAP) as an astrocyte marker, oligodendrocyte transcription factor (OLIG2) as an oligodendrocyte lineage marker and EGFR, Cyclin Dependent Kinase Inhibitor 2A (CDKN2A), Cyclin-dependent kinase 4 (CDK4) and Glioma-Associated Oncogene Family Zinc Finger 1 (GLI1) as glioma marker did not show a significant correlation with the expression of GPNMB (Additional file 4; Fig. 4D).