While hemangiomas exhibit dilated, blood-filled vascular channels lined by bland endothelial cells and stain positively for CD31, CD34, and FLI-1 (but are typically GLUT-1 negative in adults), lymphangiomas show similar spaces filled with proteinaceous fluid and are D2-40 and PROX1 positive. The gene discussed is PROX1; the disease is lymphangioma.