SLC35B4 (solute carrier family 35 member B4) is a bifunctional nucleotide sugar transporter localized to the endoplasmic reticulum and Golgi apparatus that mediates the transport of UDP-xylose and UDP-N-acetylglucosamine (UDP-GlcNAc) from the cytoplasm into the Golgi lumen 1. The protein contains a critical C-terminal dilysine motif at position 329 required for proper ER localization 2. SLC35B4 functions in glycoconjugate biosynthesis and is glucose-responsive, with expression increasing ~60% upon glucose stimulation in hepatocytes 3. Clinically, SLC35B4 has emerging relevance in multiple disease contexts. In influenza A virus infection, SLC35B4 promotes viral internalization by facilitating UDP-xylose-dependent heparan sulfate biosynthesis, which regulates AGRN proteoglycan homeostasis and affects endocytic machinery 4. In gastric cancer, SLC35B4 is a YAP1 downstream oncogenic target, with elevated expression associated with poor prognosis and enhanced cell proliferation 5. In prostate cancer, a regulatory SNP (rs1646724) in SLC35B4 correlates with increased recurrence and mortality risk, and SLC35B4 downregulation suppresses cancer cell proliferation and invasion 6. Additionally, SLC35B4 deletions have been identified in PHACE syndrome patients, though likely requiring additional genetic factors for disease manifestation 7.