The patient underwent complete endoscopic removal of the lesion; histologic examination revealed an adult-type rhabdomyoma based on the histologic features and the immunoreactivity of the neoplastic cells for desmin, myoglobin, and muscle-specific actin but not for cytokeratin, S-100, CD68R, chromogranin-A, and synaptophysin. The gene discussed is MB; the disease is rhabdomyoma.