Predominantly catalyzes the biosynthesis of ganglioside GD1alpha from GM1b in the brain, by transferring the sialyl group (N- acetyl-alpha-neuraminyl or NeuAc) from CMP-NeuAc to the GalNAc residue on the NeuAc-alpha-2,3-Gal-beta-1,3-GalNAc sequence of GM1b (PubMed:12668675). GD1alpha is a critical molecule in the communication and interaction between neuronal cells and their supportive cells, particularly in brain tissues, and functions as an adhesion molecule in the process of metastasis (By similarity). Also shows activity towards sialyl Lc4Cer (N-acetyl-alpha-neuraminosyl-(2->3)-beta-D-galactosyl- (1->3)-N-acetyl-beta-D-glucosaminyl-(1->3)-beta-D-galactosyl-(1->4)- beta-D-glucosyl-(1<->1')-N-acyl-sphing-4-enine) generating disialyl Lc4Cer, which can lead to the synthesis of disialyl Lewis a (Le(a)), suggested to be a cancer-associated antigen (PubMed:12668675). . This is the function of ST6GALNAC5 (ST6 N-acetylgalactosaminide alpha-2,6-sialyltransferase 5, ENSG00000117069).