Although the EWS/FLI-1 fusion gene is specific for ES/PNET, FLI-1 protein expression is not.     ///    "It is not surprising that some ES/ PNET are FLI-1-negative, because not all ES/PNET have the classic EWS/FLI-1, and some cases of ES/PNET may produce either low levels of protein or idiotypically different protein.. "