DDAH2 and psychiatric disorder: Regarding DDAH proteins, current data from preclinical studies can be broadly divided into 2 categories: 1) descriptive changes in DDAH1 or DDAH2 in models relevant to psychiatric disorders and 2) genetic models that manipulate DDAH1 or DDAH2 protein levels, potentially in a global, brain-specific, or cell type–specific manner (Figure 1, central circle, outer section; Table 4).