CTAG1B (cancer/testis antigen 1), also known as NY-ESO-1 (Autoimmunogenic cancer/testis antigen) is an 18 kDa protein with putative roles in germ cell self-renewal and/or differentiation. Of cancer/testis (CT) antigens, CTAG1B is the most immunogenic CT antigen known to date. CTAG1B staining is predominantly cytoplasmic with focal nuclear expression. Tissue distribution is highly restricted in the normal adult; it is only detected in spermatogonia and primary spermatocytes within the testis. Surrounding non-gametogenic cells, including Sertoli cells and spermatids are negative. CTAG1B is also undetected in the ovaries. However, expression is observed in germ cells of the fetal testis and ovaries.

In cancer, genome wide demethylation was shown to induce CTAG1B expression. Immunohistochemical expression has been demonstrated commonly in myxoid and round cell liposarcoma (89-100%), neuroblastoma (82%), synovial sarcoma (80%), melanoma (46%) and epithelial ovarian cancer (43%). Staining in lung, esophageal, liver, gastric, and bladder cancers has also been noted with lower prevalence. Immunoreactivity was only observed in tumor cells; no staining was present in adjacent cells. Furthermore, previous studies have correlated CTAG1B antigen expression with advance stage of disease and indicated a subset of more aggressive tumors that may be less susceptible to known treatments. CTAG1B antibody has been used for identification of myxoid and round cell liposarcoma.