We examined the relationship between GBA1 mutations, cathepsin D (pro- and mature CTSD) and monomeric α-synuclein levels in neural crest stem cells (NCSC)-derived dopaminergic neurons from heterozygous GBA1 mutation carriers with PD, and found reduced levels of CTSD (pro- and mature CTSD) protein and activity; and higher levels of monomeric α-synuclein. The gene discussed is CTSD; the disease is Parkinson disease.