Even though GBM tends to be a cranial-restricted tumor with distant metastases accounting 0.5–2% of all GBM patients, several research groups have demonstrated the isolation of CTCs from GBM cells, both in vivo and in vitro, with a variety of methods (glial fibrillary acidic protein-GFAP-based assay, immunomagnetic and immunofluorescence-based cell selection) [164,167,168]. The gene discussed is GFAP; the disease is neoplasm.