CD99 and Ewing sarcoma: While the widely used immunohistochemical biomarker CD99 shows high sensitivity for Ewing sarcoma, its low specificity and high expression in morphological mimics such as CIC- and BCOR-rearranged sarcomas, as well as in certain lymphoma subtypes and poorly differentiated synovial sarcoma, are problematic [3, 11–13, 16–18].