In addition to endothelial NOS (NOS3), there are also neuronal-derived NOS (nNOS, also called NOS1) and inducible NOS (iNOS, also called NOS2).97 However, in the absence of tetrahydrobiopterin (HB4) and the key enzymes GTP cyclohydroxylase 1 (GCH1) and dihydrofolate reductase (DHFR), uncoupling occurs, and superoxide O2− is produced.98,99 O2− has been found to be significantly increased in human AAA tissue. The gene discussed is NOS2; the disease is triple-A syndrome.