Unexpectedly, we identified the MLNs as a robust off-tumor IDO1 site for active immune response monitoring (figures 3 and 4), different from currently known methods that focus on tumor sites (online supplemental table 4).31–44 These findings enabled us to distinguish l-1MTrp+CPA as a more effective regimen than l-1MTrp+PTX for treating murine B16F10 melanoma (figure 3), simplifying the assessment of immunotherapy effectiveness, and allowing oncologists to rapidly analyze and screen more effective combination regimens on a common platform. Here, IDO1 is linked to melanoma.