A yolk sac tumor is a rare, malignant tumor of cells that line the yolk sac of the embryo.
These cells normally become ovaries or testes; however, the tumor can also occur in areas such as the brain or chest.
The cause of a yolk sac tumor is unknown. It is most often found in children before the ages of 1 to 2.
Yolk sac tumors are also known as germ cell tumors, teratomas, or embryonal carcinoma.