Maclura pomifera agglutinin or lectin (MPA/MPL) is tetrameric plant protein purified by affinity chromatography. MPA has a molecular weight of about 44,000 and consists of four subunits. It has an isoelectric point between pH 4.8 and pH 5.3, and blood group specificity towards A, B, and O (-SA). This lectin has a high affinity for tumor-associated T-antigen disaccharide (Gal?1,3GalNAc?) and many O-linked glycopeptide structures. The structure of Gal?1,3GalNAc? occurs on the surface of tumor cells as a mucin-associated antigenic marker, termed as T-antigen. It is also the central element in O-linked glycopeptide structures on other mammalian glycoproteins.