A yolk sac tumor is a rare, malignant tumor of cells which line the yolk sac of the embryo. These cells normally become ovaries or testes; however, the tumor can also occur in areas such as in the brain or chest. The cause of a yolk sac tumor is unknown. It is most often found in children before the ages of one to two.
Yolk sac tumors are also known as germ cell tumors, teratomas, or embryonal carcinoma.