The sequence variant c.2999T>C in exon 24 changes phenylalanine in position 1000 to serine (p.Phe1000Ser). Studies in transfected cells show that p.Phe1000 is inactive, not processed and retained in the ER. p.Phe1000 is located in the interior of the enzyme, and 3d-modelling indicates that p.Phe1000Ser may interfere with the folding process.