SSX1 and synovial sarcoma: The WHO defines SS as a mesenchymal spindle cell tumor with the possible presence of a tissue component with epithelial differentiation, including the formation of glandular structures, and with the specific pathognomonic reciprocal t(X;18) translocation, which causes the binding of SS18 with a homologous SSX gene (mainly SSX1 or SSX2, or SSX4 in rare cases).