CTSD and Parkinson disease: Plasma levels of C4 (PD vs. NC: 439.64 ± 135.04 μg/mL vs. 452.68 ± 112.13 μg/mL, p = 0.602) and cathepsin D (PD vs. NC: 261.33 ± 106.16 ng/mL vs. 226.65 ± 86.26 ng/mL, p = 0.080) were similar between patients with PD and NCs (Figure 1C,D).