A germ cell tumor is any tumor that arises from the cells that produce eggs in women and semen in men. More rarely, they can arise from such cells which, due to a congenital defect, arose outside the reproductive organs. As such, they usually form in the ovaries and the testicles. In most cases, they are benign and, although they make up the majority of tumors in these organs, make up only a small minority of tumors that mestasticize.