NGF and Alzheimer disease: Other actively investigated biomarkers for AD include nerve growth factor precursor proteins [29,30,31], which in previous studies were elevated in patients with AD in spite of the negligible difference in the nerve growth factor level compared to that in control groups, and mutations in the presenilin-1 or presenilin-2 gene [32,33,34,35], which affect the productions of Aβ40 and Aβ42 [36].