The sequence variant c.1067C>G in exon 8 changes proline in position 356 to arginine (p.Pro356Arg). Studies in transfected cells show that p.Arg356 is inactive, not processed and retained in the ER. p.Pro356 is located in the active site domain, and 3d-modelling indicates that p.Thr355Pro may interfere with formation of a disulphide-bridge and cause misfolding of the enzyme.