Proteasome inhibition resulted in elevated glucosylceramidase protein levels, as assessed by western blotting, within Gaucher disease fibroblasts [median 49% increase (IQR 30–62%), Mann-Whitney U-test P = 0.029] and Parkinson’s disease with GBA mutation fibroblasts [median increase 25% (IQR 15–35%), P = 0.043] to a greater degree than in controls (median 2% increase). Here, GBA1 is linked to Parkinson disease.