The characteristic molecular hallmark of synovial sarcoma is the chromosomal translocation t(X;18)(p11;q11), which causes the fusion of the SS18 or Synaptotagmin (SYT) gene on chromosome 18 with one of the SSX genes (SSX1, SSX2, or SSX4) on the X chromosome. The gene discussed is SS18; the disease is synovial sarcoma.