Heritable genetic variations can affect the inflammatory tumor microenvironment, which can ultimately affect cancer susceptibility and clinical outcomes. Recent evidence indicates that IDO2, a positive modifier in inflammatory disease models, is frequently upregulated in pancreatic ductal adenocarcinoma (PDAC). A unique feature of IDO2 in humans is the high prevalence of two inactivating single-nucleotide polymorphisms (SNP), which affords the opportunity to carry out loss-of-function studies directly in humans. In this study, we sought to address whether genetic loss of IDO2 may influence PDAC development and responsiveness to treatment.Experimental Design: Transgenic Ido2+/+ and Ido2-/- mice in which oncogenic KRAS is activated in pancreatic epithelial cells were evaluated for PDAC. Two patient data sets (N = 200) were evaluated for the two IDO2-inactivating SNPs together with histologic, RNA expression, and clinical survival data.