GBA1 and Parkinson disease: The fluorescence intensity of fluorescein as measured by flow cytometry has been used to assess GCase activity in peripheral immune cells of patients with Gaucher disease (Rudensky et al., 2003), a lysosomal storage disease caused by homozygous mutations in GBA1 (Hruska et al., 2008; Riboldi and Di Fonzo, 2019), as well as to demonstrate that GCase activity is reduced in monocytes from patients with PD (Atashrazm et al., 2018; Hughes et al., 2021; Wallings et al., 2022).