GBA1 and Parkinson disease: GBA1 hydrolyzes glucosylceramide into ceramide and glucose in lysosomes (Schapira, 2015), and a recent study showed that CTSB is activated by ceramides to promote PSAP cleavage to saposin C, a coactivator of GBA1 in lysosomes, and that this process is altered in Parkinson disease (Kim et al., 2022).