The present invention provides novel physiological substrates of mammalian glutaminyl cyclase (QC, EC 2.3.2.5), new effectors of QC and the use of such effectors and pharmaceutical compositions comprising such effectors for the treatment of diseases that can be treated by modulation of QC-activity, e.g. diseases selected from the group consisting of duodenal cancer with or w/o Heliobacter pylori infections, colorectal cancer, Zolliger-Ellison syndrome, Familial British Dementia and Familial Danish Dementia.