Beta-1,3-N-acetylgalactosaminyltransferase that synthesizes a unique carbohydrate structure, GalNAc-beta-1-3GlcNAc, on N- and O-glycans. Has no galactose nor galactosaminyl transferase activity toward any acceptor substrate. Involved in alpha-dystroglycan (DAG1) glycosylation: acts coordinately with GTDC2/POMGnT2 to synthesize a GalNAc-beta3-GlcNAc-beta-terminus at the 4-position of protein O-mannose in the biosynthesis of the phosphorylated O-mannosyl trisaccharide (N-acetylgalactosamine-beta-3-N-acetylglucosamine-beta-4-(phosphate-6-)mannose), a carbohydrate structure present in alpha-dystroglycan, which is required for binding laminin G-like domain-containing extracellular proteins with high affinity (By similarity). 3-O-(N-acetyl-beta-D-glucosaminyl-(1->4)-alpha-D-mannosyl)-L-threonyl-[protein] + UDP-N-acetyl-alpha-D-galactosamine = 3-O-[beta-D-GalNAc-(1->3)-beta-D-GlcNAc-(1->4)-alpha-D-Man]-L-Thr-[protein] + H(+) + UDP Protein modification; protein glycosylation. Present in testis (at protein level). In testis, it is mainly detected in the middle layers of seminiferous tubules at stages XII to II. Strongly expressed in primary and secondary spermatocytes and early round spermatids, but not in spermatogonia, elongating or elongated spermatids, or in Leydig or Sertoli cells. N-glycosylated. Belongs to the glycosyltransferase 31 family.