The rate-limiting step in this pathway is catalysed by indoleamine 2,3-dioxygenase 1 (IDO1) and by tryptophan 2,3-dioxygenase (TDO), with the IDO1 paralogue IDO2 also potentially playing a role4,5 Preclinical data suggests that a combination of IDO1 inhibitors with other anticancer agents results in effective anti-tumor immunity.6–8. This evidence concerns the gene TDO2 and neoplasm.