UAP1 is a 56KDa isoform which catalyzes the final step of UDP-N-acetylglucosamine (UDP-GlcNAc) biosynthesis from fructose-6-phosphate. UDP-GlcNAc, the activated form of GlcNAc moiety is used as a key precursor in N-linked glycosylation and the glycosylphosphatidylinositol (GPI)-anchor of cellular proteins. Crystallization studies revealed UAP1 is composed of a large central core flanked by two smaller extra domains. The central main domain displays a alpha/beta structure resembling the Rossmann fold. Northern Blot analysis detected abundant expression of UAP1 in testis and sperm with a low expression in placenta, muscle and liver. It is implicated in antibody-mediated infertility in males.