A fluorogenic substrate for α-L-iduronidase, an enzyme found in cell lysosomes that is involved in the degradation of glycosaminoglycans; cleaved by α-L-iduronidase to release the fluorescent moiety 4-MU; ex = 320 and 360 nm at low (1.97-6.72) and high (7.12-10.3) pH, respectively; em = 445-455 nm, increasing as pH decreases