Glycosyltransferase that generates the core 1 O-glycan Gal-beta1-3GalNAc-alpha1-Ser/Thr (T antigen), which is a precursor for many extended O-glycans in glycoproteins. Plays a central role in many processes, such as angiogenesis, thrombopoiesis and kidney homeostasis development. an N-acetyl-alpha-D-galactosaminyl derivative + UDP-alpha-D-galactose = a beta-D-galactosyl-(1->3)-N-acetyl-alpha-D-galactosaminyl derivative + H(+) + UDP Protein modification; protein glycosylation. Homodimer; disulfide-linked. Interacts with the C1GALT1C1 chaperone; required for galactosyltransferase activity (By similarity). Primarily expressed in endothelial, hematopoietic and epithelial cells during development. Mice develop brain hemorrhage that cause death at 14 dpc during development. They express the nonsialylated Tn antigen and brains form a chaotic microvascular network with distorted capillary lumens and defective association of endothelial cells with pericytes and extracellular matrix. Belongs to the glycosyltransferase 31 family. Beta3-Gal-T subfamily.