PENK and Alzheimer disease: The stable precursor fragments of the neuropeptides enkephalin (proenkephalin A [MR-PENK A]) and substance P (protachykinin A [NT-PTA]) have been found in blood and cerebrospinal fluid (CSF)7,8 and have recently been used for the assessment of vascular dementia, Alzheimer’s disease and neuroinflammatory disorders9.