Using a novel analytic method, termed synthetic essentiality (SE), we identified Tryptophan 2,3-dioxygenase 2 (TDO2) as a SE gene for APC deficient cancers (i.e., TDO2 is a critical downstream effector of APC deficiency). This finding may provide a basis for the costly failures of IDO1 inhibitors in cancer trials and illuminate an alternative strategy for the use of TDO2 inhibitors.