GALNS and mucopolysaccharidosis: Several assays for lysosomal enzymes were developed in a multiplex format using MS/MS combined with liquid chromatography (LC-MS/MS) [18]; a combined multiplex assay for MPS has been reported for IDUA (MPS I), iduronate-2-sulfatase (I2S; MPS II), N-acetylgalactosamine-6-sulfatase (GALNS; MPS IV), and N-acetylgalactosamine-4-sulfatase (ARSB, MPS VI) [26].