Reaction catalyzed by polypeptide N-acetylgalactosaminyltransferase (2.4.1.41)

2		+			=	2		+
	2 UDP-N-acetyl-alpha-D-galactosamine	+		polypeptide Muc1a	=		2 UDP	+		N-acetyl-alpha-D-galactosaminyl-polypeptide Muc1a