Serum tumor markers, including alphafetoprotein and human chorionic gonadotropin, are critical to the choice of therapy and monitoring of patients with germ cell tumors. i(12p) is a specific chromosomal marker for germ cell tumors that is diagnostically useful, of possible prognostic importance, and detectable by both conventional and molecular cytogenetic techniques. A variety of other molecular genetic markers are under study, in order to contribute to the understanding of normal and aberrant differentiation and to understand their relevance as tumor markers.