A tumor that develops in the reproductive tissues that become egg cells or sperm cells. Germ cell tumors are a type of ovarian cancer or testicular cancer. About 95 percent of all testicular cancers are germ cell tumors. A small group of cancers known as extragonadal germ cell tumors, which do not involve reproductive tissue, have been identified in recent years. These tumors develop in the central nervous system and account for about 2 percent of all brain tumors in the United States. Usually they occur in adolescence and young adulthood.