CTSB and Parkinson disease: In addition, Kim et al. suggested that loss of GBA1, Sphingomyelin phosphodiesterase (SMPD1), or Galactocerebrosidase (GALC) function in PD causes lysosomal ceramide deficiency; reduced ceramide-mediated cathepsin B activation in the lysosomes subsequently impairs the processing of prosaposin to saposin C, ultimately impairing GCase activity [178].