SSEA-1 (CD15, Lewis x) is a cell-surface carbohydrate antigen synthesized by FUT4 and FUT9. This carbohydrate plays a role in cell-to-cell recognition and also functions to inhibit neutrophil chemotaxis, activate normal monocytes, and mediate phagocytosis. SSEA-1 is expressed on the majority of granulocytes, up to 60% of monocytes, eosinophils, neutrophils, in proximal and distal renal tubules, and activated B and T cells. It is not present on lymphocytes. It is a useful marker for classic and follicular Hodgkin lymphoma, as it shows membranous and cytoplasmic staining in Reed-Sternberg cells. Additionally, it is positive in some peripheral T-cell lymphomas, leukemia, colorectal, lung, ovarian, peritoneal serous and renal cell carcinomas, and in mycosis fungoides, and it is a marker of poor prognosis in acute promyelocytic leukemia. It is also positive in papillary and medullary thyroid carcinoma, with higher positivity in anaplastic and conventional carcinoma over follicular variants. It may be a useful marker for malignant thyroid neoplasms as it does not typically stain benign thyroid tissues. References: Endocr Pathol. 2016 Dec;27(4):271-275, PMID: 27550342; Leuk Res 2014;38:194, PMID: 24296270; Diagnostic Immunohistochemistry: Theranostic and Genomic Applications, 3rd Edition. 2011. Ch 6: 156-188, DOI: 10.1016/B978-1-4160-5766-6.00010-8; Mol Cell Biol. 2004 May;24(10):4221-8, PMID: 5121843