This family contains UTP--glucose-1-phosphate uridylyltransferases ( 2.7.7.9 ), UDP-sugar pyrophosphorylases ( 2.7.7.64 ), UDP-N-acetylglucosamine pyrophosphorylases ( 2.7.7.23 ) and UDP-N-acetylhexosamine pyrophosphorylases, all of which catalyse the transfer of an uridylyl group.