Given the discovery of a malignant TGCT and the patient’s disseminated metastatic lymphadenopathy, in conjunction with the unique immunophenotypic characteristics of the skull base cartilaginous neoplasm, i.e., loss of SMARCB1 expression with diffuse S100 and EMA positivity, and lack of IDH1/2 mutations, the chondrosarcoma was interpreted as representing STM arising from the patient’s metastatic TGCT. The gene discussed is SMARCB1; the disease is chondrosarcoma.