The sequence variant c.1063A>C in exon 8 changes threonine in position 355 to proline (p.Thr355Pro). Studies in transfected cells show that p.Pro355 is inactive, not processed and retained in the ER. p.Thr355 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.