We demonstrated the specificity of the assay using the GCase inhibitor conduritol b-epoxide and GBA1 knockout murine neural cells and assessed GCase activity in neural stem cells and neurons differentiated from iPSC obtained from patients with PD-associated mutations LRRK2 G2019S and GBA1 N370S, as well as iPSC from PD patients without these mutations and from controls. Here, GBA1 is linked to Parkinson disease.