GFAP and central nervous system cancer: Circulating tumor-derived cells were identified by protein expression of canonical tumor markers (Epithelial: CK+; Uveal melanoma: NKI beteb+; Glioma: glial fibrillary acidic protein (GFAP+)41; Pancreatic neuroendocrine tumor (PNET): chromogranin A (CHGA+), synaptophysin (SYP+)42) using two different platforms, flow cytometry and fluorescence microscopy.