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 cause of a yolk sac tumor is unknown. It is most often found in children before the ages of 1 to 2, but can occur throughout life.

The term yolk sac tumors encompasses many types of tumors including germ cell tumors, teratomas, embryonal carcinoma, aysgermenomas, semenoma, etc. These tumors can occur in the testes, ovaries, chest, brain and other parts of the body.