The sixth mannose is added to the N-glycan precursor. This reaction occurs in the ER lumen and uses a different mannose donor (dolichyl-phosphate-mannose) than the previous steps. It has been proposed that ALG3, along with all the mannosyl- and glucosyltransferases in the N-glycan biosynthesis pathway that use dolichyl-phosphate-mannose or dolichyl-phosphate-glucose as donor, derive from duplications of a common ancestral enzyme (Oriol et al. 2002). Defects in ALG3 are associated with Congenital Disorder of Glycosylation 1D (CDG1D) (Sun et al. 2005).