GBA1 and Parkinson disease: The disease mechanisms of GBA1-related PD are yet to be understood and may not perfectly match with those of GD, as there are PD risk variants that do not cause GD in homozygosis (e.g., E326K and T369M), and since no definitive correlation has been demonstrated to date between GCase activity and GBA1-related PD [256,290,292,293,298,299].