Germ cell tumours are cancers that develop from the cells that become sperm and eggs. The most common germ cell tumours are teratomas or seminomas of the testicle. About 1 or 2% of ovarian tumours are germ cell tumours and are usually found in young women. Germ cell tumours can rarely occur in other places – for example, the pineal gland in the brain or elsewhere in the head, neck or chest.