SSX4 and synovial sarcoma: The cytogenetically defined translocation t(X; 18) (p11.2; q11.2) found in synovial sarcoma results in the fusion of the SYT gene on chromosome 18 to either SSX1, SSX2 or SSX4 on chromosome X, resulting in the formation of a SS18-SSX1, SS18-SSX2 or the rare SS18-SSX4 fusion transcript [4–7], which allow a very specific and sensitive molecular diagnosis of synovial sarcoma by fluorescence in situ hybridization (FISH) or quantitative and conventional reverse transcription-polymerase chain reaction (RT-PCR) of tumor tissue [8].