Glutaminyl cyclase (QC) catalyzes the conversion of the N-truncated Aβ peptide (Aβ3-40/42), which is obtained from Aβ40/42, into the pyroglutamate peptide AβN3pE-40/42, and its levels are elevated in the AD brain [96,97,98]. Here, QPCT is linked to Alzheimer disease.