4MU-alpha-L-idopyranosiduronic acid is a fluorogenic substrate for alpha-L-iduronidase. Active alpha-L-iduronidase cleaves the MU moiety from the synthetic substrate, which results in release of a fluorescent signal (Ex 360 nm / Em 455 nm). The reaction is stopped by the addition of sodium carbonate. 4MU-alpha-L-idopyranosiduronic acid is used in alpha-L-iduronidase assays in the context of biochemical characterisation of samples from patients affected by lysosomal storage disease mucopolysaccharidosis I (MPS I).