GFAP and neoplasm: Microscopically, two distinct neoplastic cell populations are observed in the same tumor mass with relatively clear separation between certain areas, the background fibrous tumor tissue predominantly consists of pitNET cells and the nodule portion primarily of ganglion cells.5 On immunohistochemistry (IHC), the ganglion cells are positive for synaptophysin and neurofilament protein (NFP), but negative for glial fibrillary acid protein (GFAP) while the adenomatous (pitNET) cells express synaptophysin, but not NFP.