A CT-guided biopsy of the mediastinal tumor showed the presence of tumor cells with mixed sinusoidal-like, cystic, and papillary structures (Fig. 2a) and Schiller–Duval body (Fig. 2b), which was positive by immunohistochemistry (IHC) for cytokeratin (CK) AE1/AE3 and AFP, and negative for placental alkaline phosphatase (PLAP), β-hCG, and CD30 (Fig. 2c, d), resulting in a diagnosis of primary mediastinal yolk sac tumor. The gene discussed is ALPP; the disease is neoplasm.