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 (By similarity). Interacts with the C1GALT1C1 chaperone; required for galactosyltransferase activity. Widely expressed. Highly expressed in kidney, heart, placenta and liver. Aberrant O-galactosylation of IgA1 molecules plays a role in the development and progression of IgA nephropathy (IgAN). Genetic interactions of C1GALT1 and ST6GALNAC2 variants influence IgA1 O-glycosylation, disease predisposition, and disease severity, and may contribute to the polygenic nature of IgAN. Belongs to the glycosyltransferase 31 family. Beta3-Gal-T subfamily. Core1 UDP-galactose:N-acetylgalactosamine-alpha-R beta 1,3-galactosyltransferase