Abstract: Inhibitors of fibroblast activation protein alpha (FAP) and Prolyl Oligopeptidase (POP) are disclosed, along with their use in various therapies related to conditions, diseases, and disorders involving abnormal cell proliferation such as malignancies and angiogenesis, and in neural disorders such as Alzheimer's disease. Stalk portions of the inhibitor molecules, and substrates of FAP and POP, are also disclosed and may be used, for example, in screening methods for identifying such inhibitors.