Table 2.

Cell identity approaches used in glioblastoma brain tumors

Identifying category	Feature(s)	Implications	Proof type
Morphology and location*	SVZ contact	Poor prognosis	Correlative, not functional [108]
Surface epitopes	CD44, CD133, EGF	GBM stem cells	Good evidence, not definitional [109, 110]
Transcription factors	POU3F2, SOX2, SALL2, and OLIG2	GBM stem-like, tumor propagating cells	Good evidence, functional, not definitional [21]
Functional test or assay	Self-renewal, neurosphere formation	GBM stem cells	Functional, not definitional [111]
Lineage tracing assay	Serial xenotransplantation	GSCs as tumor initiators	Functional [18, 111]
Phospho-protein signaling response	▲ Basal p-STAT5 ▲ Basal p-S6	Negative prognostic GBM cells	Definitional, functional [25]
DNA mutations	IDH1 or IDH2 mutation	Metabolically reprogrammed glioma cells	Correlative, not functional [112]
Transcription factor mRNA and genetics	Signatures including PDGFRA, IDH1, EGFR, and NF1	Classical, mesenchymal, and proneural subtypes **	Correlative, not functional [113]

^*Additional features to consider here could include location in tumor core or periphery, association with vasculature, degree of pathology-defined necrosis, and ability to form gap junctions with tumor cells or synapses with neurons. **Classification system based originally on a set of 200 + transcripts, DNA mutation, and copy number status