GAP43 and glioblastoma: Confocal imaging of spheres revealed ultra-long cellular projections consistent with tumor microtubes; this was confirmed in untreated spheres by assessing the expression of markers for GBM cell and tumor microtubes, including nestin, GAP-43, tubulin, and F-actin, consistent with GBM tumor microtubes (Figure 2A,B and Figure S3).