Interestingly, CD99 expression was confined to the cytoplasm or membrane in more malignant astrocytomas, in contrast to non-neoplastic brain tissue or non-infiltrative pilocytic astrocytoma, which showed no obvious staining in these structures.
In our preliminary experiment, we noted the absence of CD99-immunoreactivity in a fraction of brain tumors with clear cell morphology, including oligodendroglioma, clear cell ependymoma, and pilocytic astrocytoma (the oligodendroglioma-like component).