GFAP and neoplasm: We conclude that a strong correlation of GFAP to astrocytoma malignancy is absent, that the GFAP positive population is highly heterogeneous, and that distinguishing between the GFAP isoforms GFAPδ and GFAPα might improve the assessment of the differentiation state and malignancy of the tumor and identify subpopulations of GFAP expressing astrocytoma cells.