SNCA and Parkinson disease: The median concentration of α-synuclein aggregates in stool samples of iRBD patients (9.2 fM) was significantly elevated compared to those of the control group (5.2 fM, p = 0.024) as well as those of PD patients (3.8 fM, p < 0.001) when using the Kruskal–Wallis H test (Fig. 6a).