The human gene SLC35A3 encodes a UDP-GlcNAc transporter. It is ubiquitously expressed and resides on the Golgi membrane where it transports UDP- N-acetylglucosamine (UDP-GlcNAc) into the Golgi lumen in exchange for UMP. UDP-GlcNAc is a substrate required by Golgi-resident glycosyltransferases that generate branching of N-glycosylated proteins. Defects in SLC35A3 can cause arthrogryposis, mental retardation, and seizures (AMRS; MIM:615553) (Edvardson et al. 2013). Patient cells show a large reduction of tetraantennary N-glycans with an accumulation of abnormal lower-branched glycoproteins, although the serum N-glycome was normal.