SNCA and Parkinson disease: However, although a significant difference was found between the plasma levels of total form of α-synuclein between PD patients and controls (Figure 1B), the baseline total form of α-synuclein level was not associated with an increased risk of motor symptom progression after adjusting for age, sex, and baseline motor MDS-UPDRS part III scores (adjusted hazard ratio 1.08 (0.98–1.19), p = 0.09).