Abstract: Polypeptides are disclosed that are useful for diagnosing seminoma. The polypeptides have a sequence that corresponds to the amino acid sequence of the human endogenous retrovirus K10 (HERV-K10). Also disclosed are recombinant human endogenous retrovirus K10 (HERV-K10) env and gag polypeptides that are synthesized using genetic engineering techniques, constructs and processes for producing the recombinant polypeptides, and an assay for detecting the presence of antibodies specific for HERV-K10 env and/or gag polypeptides in human bodily fluids.