Glutaminyl-peptide Cyclotransferase, also known as Glutaminyl Cyclase, catalyzes the conversion of N-terminal L-glutaminyl residues of peptides to pyroglutamyl groups. The enzyme is present in the pituitary and adrenal glands, where it is important for the generation of the N-terminal pyroglutamyl groups of peptide hormones such as neurotensin and thyrotropin-releasing hormone. Glutaminyl Cyclase also catalyzes the conversion of N-terminal L-glutamyl residues to pyroglutamyl residues. This activity may contribute to the formation of several amyloid-related plaque forming peptides, contributing to Alzheimer's disease pathology. Glutaminyl Cyclase is also considered to be a diagnostic marker of thyroid tumors.