GFAP and neoplasm: While the bulk of the tumor cells appeared negative for DLK1 expression, DLK1 signal was detected in areas of GFAP staining both in perinecrotic and perivascular tumor areas, and both in GFAP positive and negative cells, suggesting DLK1 expression in astrocytes, tumor cells, and potentially other cell types present in these areas (Figure 1A−C, F−G and Supplementary Figure 1A−B).