Germ cells are the cells that become an embryo, which develops into a child in the womb. Some of these cells remain in different parts of a child's body after birth and may produce tumours. Some tumours produced by germ cells are benign and some are malignant. Germ cell tumours may appear at any age. The most common sites for germ cell tumours to develop are the testes, ovaries, in the middle of the brain, chest and abdomen and the area at the bottom of the spine.