Sox9 expression is not limited to chondroid neoplasms: variable occurrence in other soft tissue and bone tumors with frequent expression by synovial sarcomas.

The transcription factor Sox9 is known to play a crucial role in normal chondrogenesis, and antibodies against Sox9 have been proposed as a diagnostic tool for neoplasms with chondroid differentiation. However, the pattern of Sox9 immunohistochemical expression by other bone and soft tissue neoplasms, as well as its diagnostic specificity, remain unexplored. The authors have performed immunohistochemistry with antibodies against Sox9 in 106 chondroid and nonchondroid bone and soft tissue neoplasms. Moderate to intense Sox9 nuclear staining was observed in 14/20 chondrosarcomas (70%), and in 24/81 (29.6%) cases from a multitumor tissue microarray, which included 16/18 synovial sarcomas, 4/15 osteosarcomas, 2/5 peripheral primitive neuroectodermal tumor (PNET)/Ewing sarcomas, 1/1 mesenchymal chondrosarcoma, and 1/1 chondroblastoma. The results suggest that Sox9 usefulness in the diagnosis of chondroid tumors may be limited because of low sensitivity and specificity. The finding of Sox9 expression by 88.9% of synovial sarcomas represents a novel and striking observation, which deserves further investigation.