IDO1 and fungal infectious disease: In particular, the Kyn pathway has been involved in balancing resistance and tolerance to fungal infections (18) and decreased Trp catabolism, as a result of IDO1 deficiency, was causally linked to susceptibility to Aspergillus fumigatus infection in murine models of cystic fibrosis (CF), a monogenic autosomal recessive disorder in which the mutation in the CF transmembrane conductance regulator (CFTR) results in an exaggerated but ineffective airway inflammatory response that fails to eradicate pathogenic fungi (19, 20).