3,4-Bis-O-[(1,1-dimethylethyl)dimethylsilyl]-1,2-O-[1-1-methoxyethylidene]-B-L-idopyranosiduronic Acid Methyl Ester is an intermediate in the synthesis of fluorogenic substrate in the assay of α-L-Iduronidase. A fluorimetric enzyme assay for the diagnosis of MPS II (Hunter disease).