CD99 and primitive neuroectodermal tumor: Schmidt et al (15) proposed the following criteria for the pathological diagnosis of PNET: a) Tumor cells appear round under a light microscope and Homer-Wright-type rosettes are present; b) immunohistochemistry reveals that the tumor is positive for 013 (i.e., HBA71, P30/32 and MIC-2), and for two or more of the NSE, Syn and CGA proteins; and c) ultrastructural identification of neural elements, especially neurosecretory granules (14,16).