We emphasize the differential diagnoses engendered by ASPS including a series of tumors involving the lung that have nested and alveolar growth patterns, and both clear and eosinophilic cytoplasm, and demonstrate the utility of molecular genetic analysis for TFE3 rearrangement and immunohistochemistry for TFE3 antigen expression for arriving at accurate diagnosis. The gene discussed is TFE3; the disease is alveolar soft part sarcoma.