OLIG2 and glioma: Using established neuropathological criteria for pediatric LGGs [10], we defined a LGG as (1) a mass-occupying lesion with architectural distortion by standard H&E staining (and by MRI in a subset of cases), with (2) increased proliferation (Ki67 labeling index > 1%) and (3) immunopositivity for glial immunohistochemical markers used in the routine diagnosis of human low-grade gliomas (e.g., GFAP and OLIG2).