NY-ESO-1, a member of the CT (cancer/testis) family, is a tumor-specific shared antigen with distinctive immunogenicity. The molecular basis of T cell-mediated antitumor immunity has been elucidated by the identification of a number of tumor antigens recognized by CD8+ T cells. Among these antigens, NY-ESO-1 is of particular interest because both the cytotoxic T lymphocyte and antibody have been shown to react with this antigen. The NY-ESO- 1 has been detected in normal testis and in a range of human tumor types. NY-ESO-1 reactivity in testis is restricted to germ cells, particularly in the spermatogonia.