GCH1 and Cognitive impairment: As indicated in Fig. 7A, Table 3, and Table 4, HTL significantly induced multiple deficits of learning and memory in mice infected with AAV9 expressing WT-GCH1, but not in mice infected with AAV9 harboring MT-GCH1, demonstrating that HHcy via GCH1 S-nitrosylation at C141 induces cognitive impairments in vivo.