Negative results for DOG-1, CD117 (KIT), CD34, S-100 protein, neurofilament, cytokeratin, epithelial membrane antigen (EMA), and ALK suggest that PF is a distinct disease entity from GIST, angiomyxoma, neurogenic tumor, sarcomatoid carcinoma, and inflammatory myofibroblastic tumor. The gene discussed is ALK; the disease is inflammatory myofibroblastic tumor.