Abstract: Structural coordinates obtainable by subjecting a crystal comprising a dimethylarginine dimethylaminohydrolase (DDAH) or a fragment thereof or an argininc deliminase (DI) or a fragment thercof to X-ray diffraction measurements and deducing the structural coordinales from the diffraction measurements arc used to identify, screen, characteristic, design or modify a chemical entity. The chemical entities so generated may be used in methods of Therapy or to identify the presence or absence of a DDAH or DI substrate.