PMM2-CDG is caused by mutations in the PMM2 gene. This gene provides instructions for making an enzyme called phosphomannomutase 2 (PMM2). Mutations in the PMM2 gene lead to the production of a malfunctioning or non-functioning PMM2 enzyme. The PMM2 enzyme is involved in a process called glycosylation, which attaches groups of sugar molecules (oligosaccharides) to proteins. Without a properly functioning PMM2 enzyme, glycosylation cannot proceed normally in the body.