Phosphomannomutase 2 (PMM2) normally catalyses the isomerisation of mannose 6-phosphate (Man6P) to mannose 1-phosphate (Man1P) in the cytosol of cells. Man1P is a precursor in the synthesis of GDP-mannose and dolichol-phosphate-mannose, required for critical mannosyl transfer reactions in the N-glycosylation of proteins. Mutations in the PMM2 gene are one of the causes of Jaeken syndrome, a congenital disorder of glycosylation type 1a (PMM2-CDG, previously CDG-1a) (Matthijs et al. 1997). PMM2-CDG was first described in Belgian identical twin sisters, characterized by psychomotor retardation and multiple serum glycoprotein abnormalities. Serum and CSF transferrin were found to be deficient in sialic acid (Jaeken et al. 1984). PMM2-CDG is the most common CDG disease subtype.