Amyloid-β peptides having pyroglutamate at the third residue (3pE Αβ) are a major species deposited in the brain of AD patients. 3pE Αβ is present in almost all diffuse and mature plaques in AD, is metabolically stable and may play a role in both plaque seeding and stabilization. Detectable amounts of 3pE Αβ have not been reported in CSF or plasma, thus suggesting that the target peptide is pathology specific.