The circulating levels of plasma MMP-9 were more elevated in clinical AD (N = 30; Mean age: 71 ± 11 years) than in the HC (N = 32; Mean age: 61 ± 18 years), PD (N = 24; Mean age: 67 ± 10 years), and ALS (N = 35; Mean age: 50 ± 16 years) patients from Massachusetts General Hospital and Weill Medical College of Cornell University [67], which suggested that plasma MMP-9 could be a suitable biomarker to differentiate other NDs from AD. The gene discussed is MMP9; the disease is Alzheimer disease.