DAO and neoplasm: To understand the role of H2S in oncogenesis and tumor progression, it is important to examine the complex interactions between the enzymes that synthesize H2S (such as CBS, CSE, 3-MST, cysteine aminotransferase [CAT], D-amino acid oxidase [DAO], and cysteinyl-tRNA synthetase 2 [CARS2]) and those involved in its catabolism (the sulfide oxidation unit [SOU], which includes sulfide–quinone oxidoreductase [SQOR], human ethylmalonic encephalopathy protein 1 [hETHE1], rhodanese, sulfite oxidase [SUOX/SO], and cytochrome c oxidase [CcO]) [10,11,12,13].