MZ-101 (GYS1-IN-2) is an orally active, potent and selective small-molecule glycogen synthase 1 ( GYS1 ) inhibitor with with an IC 50 value of 0.041 µM. MZ-101 reduces glycogen concentrations in cells and in mice. MZ-101 can used to study GYS1 -mediated Pompe disease and other glycogen storage diseases.