GFAP and glioblastoma: In one study, blood cell samples from GBM patients were screened by immunostaining for glial fibrillary acidic protein (GFAP) as a GBM marker, and GFAP-positive cells harboring glioblastoma-associated genomic aberrations were present in 29 out of 141 (21%) GBM patients, not in healthy volunteers48.