SLC35D1 encodes an endoplasmic reticulum antiporter that transports nucleotide sugars—including UDP-GlcNAc, UDP-GalNAc, and UDP-GlcA—across the ER membrane in exchange for their cognate nucleoside monophosphates or other nucleotide sugars. These substrates are essential cofactors for ER glucosyltransferases that synthesize glycoprotein and glycolipid sugar chains. The transporter also couples UDP-GlcNAc or UDP-GalNAc efflux to UDP-GlcA influx, facilitating glucuronidation and excretion of endogenous and foreign compounds. SLC35D1 has a critical role in chondroitin sulfate biosynthesis, which is essential for cartilage extracellular matrix formation and skeletal development 1. Loss-of-function mutations cause Schneckenbecken dysplasia, an autosomal recessive lethal skeletal dysplasia characterized by hypoplastic iliac bones with a snail-like configuration, vertebral flattening, and severe limb shortening 12. Mice deficient in Slc35d1 develop lethal skeletal dysplasia with reduced chondroitin sulfate chain length and defective proteoglycan aggregation 1. Notably, hypomorphic alleles produce milder phenotypes; a missense variant (p.Met134Thr) was identified in a family with affected individuals ranging from 4 to 31 years old, representing the first survival beyond the perinatal period 3. Genome-wide association studies have implicated SLC35D1 as a potential risk gene for leprosy, with genetic variants affecting mRNA expression and differential expression observed in skin tissues of leprosy patients 4.