We measured total and active MMP-2/-9 and additional biomarkers (creatinine, neurofilament light, cystatin C, and alkaline phosphatase) in the serum of people with ALS (ALS, n = 30) and compared their levels with age-matched healthy controls (HC, n = 20) and other neurological diseases (diabetic nephropathy, Alzheimer’s disease, Parkinson’s disease; n = 8 each). The gene discussed is CST3; the disease is diabetic kidney disease.