4-Methylumbelliferyl-2-acetamido-2-deoxy-β-D-glucopyranoside is a fluorogenic substrate for β-hexosaminidases. Upon enzymatic cleavage by β-hexosaminidases, 4-methylumbelliferone (4-μU) is released, and its fluorescence quantifies β-hexosaminidase activity. 4-MU fluorescence is pH-dependent with excitation maxima of 320 and 360 nm at low (1.97-6.72) and high pH (7.12-10.3), respectively, and an emission maximum ranging from 445 to 455 nm, increasing as pH decreases. This compound has been used to quantify β-hexosaminidase activity in serum or leukocytes from patients with GM2 gangliosidoses, such as Tay-Sachs disease, which is characterized by defects in the α subunit of β-hexosaminidase.