Although less studied, IDO2 has been found to be overexpressed in some human tumors, to functionally enable IDO1-dependent Treg suppression (Metz et al. 2014), and to underpin B cell–mediated autoantibody production (Merlo et al. 2016), which is important in the development of certain cancers, such as squamous cell carcinomas (Prendergast et al. 2017). This evidence concerns the gene IDO1 and cancer.