More than 90% of our cases stained positive for CD99; however, unlike the strong and homogeneous membranous staining typically seen in Ewing’s sarcoma (or primitive neuroectodermal tumor), the CD99 staining in synovial sarcoma is usually weaker and more diffuse with both cytoplasmic and membranous distribution. This evidence concerns the gene CD99 and Ewing sarcoma.