is rare and aggressive soft tissue sarcoma that appears as deep, painless, slowly growing masses. Most frequently the tumor affects the legs, especially the knees, and the arms. Less frequently, this disease develops in the trunk, head and neck region, or the abdomen. Although synovial sarcoma does not have a clearly defined cause, genetic factors are believed to influence the development of this disease. Cells in these tumors are usually characterized by the presence of a translocation involving chromosomes X and 18. This translocation is specific to synovial sarcoma and is often used to diagnose this cancer. Although this condition generally affects adults, about 30% of reported cases occur in children and adolescents. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.