Carbazole 1,9a-dioxygenase (, CARDO) is an enzyme with systematic name 9H-carbazole,NAD(P)H:oxygen oxidoreductase (2,3-hydroxylating). This enzyme catalyses the following chemical reaction
The four substrates of the enzyme are carbazole, reduced nicotinamide adenine dinucleotide (NADH), oxygen, and a proton. Its products are 2'-aminobiphenyl-2,3-diol and oxidised NAD<sup>+</sup>. It is an oxidoreductase that uses molecular oxygen as oxidant and incorporates both its atoms into the starting material. It catalyses the first reaction in the pathway of carbazole degradation.