AimsSOX11 is known as an essential transcription factor for regulating neurogenesis. Recently, SOX11 has been suggested to be a diagnostic marker and oncogene because of its significant expression in mantle cell lymphoma (MCL). However, SOX11 expression in other tumour types has not yet been extensively studied. Methods and resultsHerein, we examined SOX11 expression in 2026 cases of neuroectodermal, germ cell, mesenchymal and epithelial tumours by immunohistochemistry. SOX11 was consistently expressed in all neuroectodermal tumours with neural differentiation, as well as in immature teratomas revealing neurogenesis. Less frequently, SOX11 expression was observed in only 50% of astrocytomas and 24% of malignant peripheral nerve sheath tumours, and was mainly sporadic and weak/intermediate. In epithelial tumours, significant SOX11 expression was identified in 97% of salivary ductal carcinomas (SDCs) and a high percentage of high-grade neuroendocrine carcinomas (NECs), especially the small-cell lung carcinomas (68%), and was absent in most other carcinomas, except for less and/or focal and weak expression in adenocarcinomas from the lung, genital tract and breast, and salivary adenoid cystic carcinomas and epithelial-myoepithelial carcinomas. In mesenchymal tumours, in addition to MCLs, prominent SOX11 expression was observed in 90% of rhabdomyosarcomas and all myxoid/round cell liposarcomas (MRCLs). Less frequent and/or focal and weak expression was observed in lymphoblastic, Burkitt and follicular lymphomas, synovial sarcoma and angiosarcoma. ConclusionSOX11 showed prominent expression in neuroectodermal tumours with neural differentiation, high grade-NEC, SDC, rhabdomyosarcoma and MRCL. The high sensitivity and specificity of SOX11 in SDC and MRCL make it a useful diagnostic marker.