SLC35D1 is an endoplasmic reticulum (ER) nucleotide-sugar antiporter that exchanges UDP-sugars for nucleoside monophosphates or other nucleotide sugars 123. It transports multiple UDP-sugars including UDP-GlcNAc, UDP-GalNAc, and UDP-GlcA 412, which serve as sugar donors for ER glucosyltransferases synthesizing glycoproteins, glycolipids, and oligosaccharides 53. SLC35D1 plays a critical role in chondroitin sulfate biosynthesis, essential for cartilage extracellular matrix formation and skeletal development 5. The transporter may also couple UDP-GlcNAc or UDP-GalNAc efflux to UDP-GlcA influx, facilitating glucuronidation and excretion of endobiotics and xenobiotics 12. Loss-of-function mutations in SLC35D1 cause Schneckenbecken dysplasia, a severe autosomal recessive skeletal dysplasia characterized by limb shortening and facial structure abnormalities 53. Affected individuals exhibit defective chondroitin sulfate chains with shortened and sparse molecules 5. Hypomorphic alleles produce milder phenotypes with variable severity depending on residual transport activity 36. Additionally, GWAS fine-mapping identified SLC35D1 as a potential leprosy susceptibility gene, with disease-associated variants affecting mRNA expression and differential expression in leprosy patient skin tissues 7. The transporter's broad substrate specificity suggests roles beyond skeletal development in various ER glycosylation reactions 3.