In addition to the advantages indicated for a good AD biomarker that are, briefly: i) being minimally invasive, ii) being widely accessible, iii) being cost-effective, and iv) being able to reproduce a relevant pathophysiological process of the disease, our study suggests that the measurement of serum BACE1 activity could be employed with beneficial effects for patient classification. The gene discussed is BACE1; the disease is Alzheimer disease.