The mucin-type O-glycome in cancer aberrantly expresses the truncated glycans Tn (GalNAcα1-Ser/Thr) and STn (Neu5Acα2,6GalNAcα1Ser/Thr). However, the role of Tn and STn in cancer and other diseases is not well understood. Our recent discovery of the self-binding properties (carbohydrate-carbohydrate interactions) of Tn (Tn – Tn) and STn (STn – STn) provides a model for their possible roles in cellular transformation. We also review evidence that Tn and STn are members of a larger family of glycan tumor antigens that possess carbohydrate-carbohydrate interactions, which may participate in oncogenesis.