Mu-6S-Palm-β-Glc is a fluorogenic substrate of palmitoyl-protein thioesterase (PPT, also known as CLN1), a lysosomal hydrolase that removes long-chain fatty acyl groups from modified cysteine residues in proteins. Mu-6S-Palm-β-Glc is cleaved by PPT/CLN1 to release the fluorescent moiety 4-methylumbelliferyl (4-MU). 4-MU fluorescence is pH-dependent with excitation maxima of 320 and 360 nm at low (1.97-6.72) and high (7.12-10.3) pH, respectively, and an emission maximum ranging from 445 to 455 nM, increasing as pH decreases. This substrate is used in assays that measure PPT activity, which is commonly deficient in the neurodegenerative disorder known as infantile neuronal ceroid lipofuscinosis.