OLIG2 and brain stem glioma: It is intriguing to note that iGRPs give rise to more compact tumors with stronger GFAP than OLIG2 immunoreactivity, resembling optic pathway and brainstem gliomas, while the iOPC-derived lesions have looser stroma with microcystic changes and a higher density of OLIG2+ than GFAP+ cells, similar to many cerebellar human PAs.