Indoleamine 2,3-dioxygenase (EC 1.13.11.42) (IDO) [1] is an enzyme responsible for the conversion of tryptophan and other indole derivatives to kynurenines. In mammals, IDO is induced by interferon-γ (IFN-γ) and responsible for some of its anti-proliferative effects. The degradative action of IDO on tryptophan leads to cell death by starvation of this essential and relatively scarce amino acid. IDO is a heme-containing enzyme of about 400 amino acids.

Other proteins that are evolutionarily related to IDO are:

• Myoglobin from the red muscle of the gastropods Nordotis madaka and Sulculus diversicolor [2]. These unusual globins lack enzymatic activity but have kept the heme group.
• Yeast hypothetical protein YJR078w.

As a signature for the IDO family, we selected two conserved regions. The first is located in the central section and is rich in glycine and serine residues; the second is located in the second third of the sequence and contains a conserved histidine that could serve as a proximal ligand to the heme iron.