The histologic features of yolk sac (endodermal sinus) tumors are characterized by several patterns with varying cytology, making histological identification sometimes difficult. The majority of yolk sac tumors show more than 1 histological type, though none are believed to have specific prognostic significance. Generally, however, yolk sac tumors are characterized by primitive tubular or "microscopic" structures lined by somewhat flattened primitive cuboidal epithelial cells. One of the most common histological variants is the "papillary type," in which the characteristic Schiller-Dural bodies are seen, as in the present case. Periodic acid–Schiff (PAS)–positive hyaline droplets can be frequently recognized in the cytoplasm of many of these cells. Immunohistochemically, these tumors characteristically stain for α-fetoprotein. Yolk sac tumor is the most common malignant germ cell tumor in prepubertal children and may be histologically differentiated from embryonal carcinoma on the basis of virtual nonexistence of the latter in young children. Yolk sac tumor is commonly, but not always, associated with elevated serum α-fetoprotein levels.