Functional positron emission tomography (PET) imaging is a potentially ideal tool for capturing real-time variations in IDO1 content, because it enables noninvasive, highly sensitive, repetitive, and quantitative imaging of positron-emitting, target-specific probes.14 The utilization of PET imaging to study the relationship between IDO1 expression and immunotherapy efficacy has never been reported, despite several IDO1 inhibitors having been radiolabeled for use in pharmacokinetic studies15 16 and to perform IDO1-targeted cancer imaging.17 18. The gene discussed is IDO1; the disease is cancer.