The ALG9 gene encodes a protein called alpha-1,2-mannosyltransferase that functions in lipid-linked oligosaccharide assembly. Specifically, this protein catalyzes two steps in the N-linked pathway of lipid-linked oligosaccharide biosynthesis. The N-linked pathway is a complex multi enzymatic process that involves two major steps. The first of these steps takes place on the cytosolic side of the ER with the production of a dolichol pyrophosphate-linked heptasaccharide. This metabolite is then translocated to the luminal side of the ER where four mannose residues and three glucose residues are added to the lipid-linked precursor oligosaccharide. The ALG9 protein catalyzes the addition of two different mannose residues, the seventh mannose residue on the B-arm as well as the ninth mannose residue on the C-arm to the lipid-linked oligosaccharide.
Mutations in the ALG9 gene are associated with two different kinds of disorders. Congenital disorder of glycosylation, type Il is a disorder that results in progressive microcephaly, hypotonia, developmental delay, drug-resistant infantile epilepsy, and hepatomegaly. The other disorder linked to this gene is Gillessen-Kaesbach–Nishimura skeletal dysplasia, a rare lethal autosomal recessive syndrome with skeletal dysplasia and visceral abnormalities. In addition, variants involving the ALG9 gene have been shown to be associated with a high frequency of bipolar affective disorder in some families.