NOS1 and neoplasm: On the other hand, genes with decreased expression in tumor tissue were AMT, which is involved in glycine cleavage system; GLUL, which catalyzes the synthesis of glutamine from glutamate and ammonia; NOS1, nitric oxide synthase, which synthesizes nitric oxide from L-arginine; PIPOX, which is involved in L-lysine catabolic process; and PRODH, which catalyzes the first step in proline degradation.