Differential diagnosis includes other histiocytic and dendritic cell neoplasms, fibroblastic reticulum cell sarcoma (expresses desmin, SMA, and cytokeratins), inflammatory pseudotumor (expression of FDC cell markers is weak), myofibroblastic tumors (positive for muscle specific actin), lymphoepithelioma-like carcinoma, malignant melanoma, thymoma, meningioma, malignant fibrous histiocytoma, and gastrointestinal stromal tumor. The gene discussed is SMN1; the disease is meningioma.