Cancer/testis (CT) antigens including MAGEA, NY-ESO-1, GAGE, CT7, CT10, CT45, SAGE1, and NXF2 are protein antigens that are normally expressed in the germ cells of adult testis and developing fetal testis and ovary, but not in any other adult tissues. Examination of various types of human cancer showed CT gene activation and protein expression in a proportion of human cancers in a lineage-unrelated fashion. Due to this restricted pattern of expression, CT antigens are often recognized by the immune system of cancer patients, and this spontaneous immunogenicity raises the possibility of their use as therapeutic cancer vaccine targets.