A germ cell is a type of cell that gives rise to the gametes (sperm and eggs) of an organism that reproduces sexually. Germ cells develop in the midline of an embryo before settling into the reproductive organs. In many animals, germ cells originate in the primitive streak and migrate via the gut of an embryo. Germ cell tumors are a rare type of cancer that can affect people of all ages. They can be benign or malignant and can occur anywhere in the body where there are germ cells. The most common sites for germ cell tumors outside of the reproductive organs are the chest, back of the tummy, brain, and lower spine. Doctors use different names to describe the different types of germ cell tumors, which can start in the testicles, ovaries, or extragonadal areas.