Using positron-emission tomography (PET), progressive BBB impairment has been shown in the midbrain of PD patients as seen by an increased uptake of the tracer 11C-verapamil indicating impairment of the BBB efflux pump P-glycoprotein (Kortekaas et al., 2005; Bartels et al., 2008) and analysis of dynamic contrast-enhanced magnetic resonance images revealed higher BBB leakage in PD patients (Al-Bachari et al., 2020), whereas a study using rubidium-82-PET could not detect BBB leakage in PD patients (Fujita et al., 2021). This evidence concerns the gene ABCB1 and Parkinson disease.