Production of HERV-K virus-like particle was identified in teratocarcinoma cell lines (Boller et al., 1993; Löwer et al., 1993); HERV-K Gag and Env proteins were expressed in germ cell tumors and interestingly antibodies against these proteins were also detected (Sauter et al., 1995, 1996). The gene discussed is ERVW-1; the disease is testicular germ cell tumor.