CD99 and primitive neuroectodermal tumor: These authors proposed the following algorithm: (1) the diagnosis of EWS/PNET is confirmed when CD99/FLI-1p is positive in the absence of other IHC markers (except focal positivity for total cytokeratin and neuroendocrine markers) and when the fluorescence in situ hybridization (FISH) (22q12) probe is rearranged; (2) the original diagnosis of EWS/PNET should be retained when CD99/FLI-1p is positive and when (22q12) FISH presents a split signal (not rearranged); and (3) the diagnosis of EWS/PNET is excluded when CD99/FLI-1p is negative and the FISH (22q12) probe is not rearranged.