ALG9 is an alpha-1,2-mannosyltransferase that catalyzes the sequential addition of mannose residues during dolichol-linked oligosaccharide biosynthesis, specifically adding the seventh and ninth alpha-1,2-linked mannose residues to Man(6)GlcNAc(2)-PP-dolichol and Man(8)GlcNAc(2)-PP-dolichol intermediates in the endoplasmic reticulum lumen 1. These lipid-linked oligosaccharides serve as glycan precursors for protein N-linked glycosylation, with the assembled 14-sugar oligosaccharides subsequently transferred to nascent proteins by oligosaccharyltransferases. ALG9 deficiency causes congenital disorder of glycosylation type IL (CDG-IL), characterized by accumulation of incomplete lipid-linked oligosaccharide intermediates and transfer of underglycosylated proteins 1. Biallelic ALG9 mutations present with severe neurological manifestations including developmental delay, seizures, hypotonia, and West syndrome 2. Monoallelic ALG9 loss-of-function variants are associated with autosomal dominant polycystic kidney disease (ADPKD) and autosomal dominant polycystic liver disease (ADPLD) 3, representing minor ADPKD genes (~0.6% of familial cystic disease) 4. The kidney phenotype is typically mild with limited progression to kidney failure 4, while liver cysts can develop as an extrarenal manifestation 5. Somatic loss of heterozygosity in hepatic cyst tissue suggests a two-hit mechanism in ADPLD pathogenesis 5. Additionally, ALG9 variants have been reported in cardiac manifestations of inherited carbohydrate metabolism disorders 6.