FCSK encodes fucokinase, an enzyme that catalyzes the phosphorylation of L-fucose to fucose-1-phosphate, serving a critical role in the fucose salvage pathway for glycoprotein fucosylation 12. The enzyme is essential for maintaining proper protein glycosylation, particularly affecting O-fucosylation of proteins like NOTCH3 3. FCSK deficiency causes FCSK-congenital disorder of glycosylation (FCSK-CDG), a rare autosomal recessive disorder characterized by intellectual disability, growth delay, seizures, feeding difficulties, and ophthalmic abnormalities 14. The disease appears to primarily affect secretory glycoproteins rather than cell surface glycans 3. Animal studies using fcsk knockout zebrafish demonstrate neurodevelopmental abnormalities including impaired growth, brain atrophy, locomotor deficits, increased seizure susceptibility, and memory impairment 2. These defects can be rescued by GDP-L-fucose supplementation, confirming the enzyme's role in fucosylation 2. Currently, only a handful of patients with FCSK-CDG have been reported worldwide, with phenotypes ranging from severe developmental delays to milder presentations 5. The rarity of this condition highlights the critical importance of fucose metabolism in human development and neurological function.