This entry includes UDP-sugar phosphorylases such as UDP-N-acetylglucosamine pyrophosphorylase, UDP-N-acetylhexosamine pyrophosphorylase (UAP1) and UDP-N-acetylglucosamine diphosphorylase, all of which catalyse the transfer of an uridylyl group. UAP1 has two catalytic domains, UDP-N-acetylgalactosamine pyrophosphorylase () and UDP-N-acetylglucosamine pyrophosphorylase () []. UDP-N-acetylglucosamine diphosphorylase is required for the biosynthesis of UDP-glucosamine, an essential precursor for glycoprotein and glycolipid synthesis and can utilize UDP-glucosamine or UDP-galactosamine [].