An isozyme of human sperm-specific lactate dehydrogenase, LDH-X (C4) (EC 1.1.1.27), which is expressed only in differentiated germ cells (spermatozoa, spermatids and primary spermatocytes after midpachytene), appeared in the xenografted tumor cells of human seminoma and its metastatic lesions (lymph node and kidney) in scid (severe combined immunodeficiency)-nude(streaker) double mutant mice, though it was not expressed in the original tumor of the patient.