IDS and mucopolysaccharidosis type 2: Mucopolysaccharidosis type II (MPS II, Hunter syndrome, Online Mendelian Inheritance in Man number 309900) is an X-linked, recessive disease that is characterised by deficiency in the activity of the lysosomal enzyme iduronate-2-sulfatase (I2S), owing to a mutation in the I2S gene (IDS) [1,2].