BACKGROUND: High-throughput gene expression studies on glioblastoma have identified proneural (PN) gene-expression type as the one characterized by a strong expression of neurogenesis-related genes. Although the PN type is generally associated with good prognosis, the clinical and biological significance of the PN type has not been completely elucidated. In the current study, we aimed to expound the biological significance of the PN subtype by quantifying its gene signature in glioblastoma, low-grade gliomas, and glioblastoma-derived cellular spheres. METHODS: To evaluate the PN gene-expression signature of all the glioma grades, we quantified the expression signature of 82 gliomas (grade II, 14 gliomas; III, 16; and IV, 52) by using real-time reverse transcription PCR. All samples had been assessed for IDH mutation and 1p/19q co-deletion. We characterized the PN signature by estimating the relative expression score of nine PN markers (DLL3, BCAN, OLIG2, NKX2.2, ASCL1, SOX2, ERBB3, NCAM1, and NCAM2) and eight mesenchymal (MES) markers (YKL-40, CD44, Vimentin, COL1A1, TRL2, TRL4, TRADD, and RELB). In addition, we evaluated the expression signature of five matched glioblastoma tissues and glioblastoma-derived cellular spheres. RESULTS: The PN gene-expression signature was higher in low-grade gliomas than that in glioblastoma. In the IDH-wild glioblastoma (n = 42), 19% of the tumor was characteristic of the PN type, while 32% was of the MES type. The overall survival time was not significantly different between two types. The analysis of five glioblastoma-derived spheres demonstrated that two of them showed the PN signature, and the remaining three showed the MES signature. Glioblastoma-derived spheres showed a different expression signature from that of the originating tumor. In particular, two spheres derived from the MES glioblastoma showed the PN signature after successful expansion as neurospheres, suggesting a conversion of the MES to PN type under proliferating conditions. CONCLUSIONS: The expression analysis of glioma tissues showed that the PN signature is displayed early in gliomagenesis, and is not specific to a subset of glioblastomas. Given that the MES to PN conversion is a frequent event under proliferating conditions, a display of the subtype-specific gene signature might be reversible phenomenon. SECONDARY CATEGORY: Neuropathology & Tumor Biomarkers.