Other tumors, such as carcinomas, melanomas, and soft tissue sarcomas, primary or metastatic, can be confused with HS and were excluded in our case by the lack of immunoreactivity for pancytokeratin, CAM 5.2, CK 903, TTF1, HMB45, MART-1/Melan A, CD99, CD117, smooth muscle actin, desmin, and myogenin. The gene discussed is CD99; the disease is histiocytic sarcoma.