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. 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. In stage 12 embryos, expression is seen in the amnioserosa and by stage 16, also in the central nervous system, predominantly the ventral nerve cord and brain. Also in embryos, preferential expression is seen in the ejaculatory duct and the nurse cells (but not the oocyte). In larvae, expression is seen throughout the nervous system and imaginal disks. Adult females show expression in the nurse cells and adult males in the ejaculatory duct. Expressed both maternally and zygotically in the embryo and throughout development. Morphogenetic defects in which the ventral nerve cord is greatly elongated and the brain hemispheres are misshapen. Belongs to the glycosyltransferase 31 family. Beta3-Gal-T subfamily.