CD99 (MIC2) is a valuable screening marker and is often strongly positive in reported mediastinal cases, but it is neither fully specific (e.g., it can be positive in synovial sarcoma, lymphoblastic lymphoma, ependymoma, and other small round blue cell tumors) nor fully sensitive (rare Ewing sarcoma cases may show only weak or even negative CD99 staining). The gene discussed is CD99; the disease is ependymoma.