A tumor-associated antigen belonging to the family of immunogenic testicular proteins that are aberrantly expressed in human cancers in a lineage-nonspecific fashion. Reverse transcription-PCR analysis showed NY-ESO-1 mRNA expression in a variable proportion of a wide array of human cancers, including melanoma, breast cancer, bladder cancer, prostate cancer, and hepatocellular carcinoma; and restricted expression in normal tissues, with high-level mRNA expression found only in testis and ovary tissues. The gene for NY-ESO-1 maps to Xq28 and codes for an 18-kDa protein having no homology with any known protein. NY-ESO-1 elicits a strong, integrated humoral and cellular immune response in a high proportion of patients with NY-ESO-1-expressing tumors and is under investigation as a cancer immunotherapy agent.