MMP9 and Parkinson disease: Compared with the control group, MMP9 level of early-stage PD patients [503 (390, 729) vs. 329 (229, 473) ng/ml; P < 0.001] and MMP9 level of advanced-stage PD patients [564 (504, 769) vs. 329.13 (229, 473) ng/ml; P < 0.001] were significantly higher.