To assess the feasibility and reliability of the molecular detection of the transcript originating from the chimeric gene in paraffin-embedded tumor specimens, we performed a nested reverse transcription-polymerase chain reaction (RT-PCR)-based assay to detect the EWS-FLI1 chimeric message in a series of Ewing family tumors. Of 24 paraffin-embedded tumor specimens from 23 cases analyzed, the chimeric message was detectable in 20 (83%) specimens from 20 cases (87%) by this nested RT-PCR assay, whereas none of 7 small round cell tumors not from this family (3 alveolar rhabdomyosarcomas, 2 neuroblastomas, 2 malignant lymphomas) showed detectable chimeric messages