Polyagglutination refers to red blood cells that agglutinate upon exposure to almost all human sera, but not to autologous serum or the sera of newborns. The condition becomes apparent during blood typing and cross-matching in the laboratory (summary by Beck, 2000). Tn polyagglutination syndrome is an acquired clonal disorder characterized by the polyagglutination of red blood cells by naturally occurring anti-Tn antibodies following exposure of the Tn antigen on the surface of erythrocytes. Only a subset of red cells express the antigen, which can also be expressed on platelets and leukocytes. This condition may occur in healthy individuals who manifest asymptomatic anemia, leukopenia, or thrombocytopenia; however, there is also an association between the Tn antigen and leukemia or myelodysplastic disorders. The Tn antigen is an incompletely glycosylated membrane glycoprotein with an exposed N-acetylgalactosamine residue. The Tn antigen results from inactivation of C1GALT1C1, which encodes a chaperone required for the correct functioning of T-synthetase (C1GALT1; 610555), an enzyme essential for the correct biosynthesis of O-glycans. Absence of active T-synthetase results in exposure of GalNAc residues, with a proportion of these residues becoming sialylated and forming a sialyl-Tn antigen (summary by Vainchenker et al., 1985 and Crew et al., 2008). [from OMIM]