l-1MTrp is a well-known IDO1 inhibitor showing specific binding to the catalytic pocket of IDO1, and has been indicated to inhibit tumor growth in diverse tumor types in vivo (figure 1A).20 Radiotracer 11C-l-1MTrp was synthesized with satisfactory radio-characteristics (figure 1B and C).16 To study the potential of 11C-l-1MTrp to specifically visualize IDO1 in human tumors, we performed 11C-l-1MTrp PET imaging in mice inoculated with human tumors with different IDO1 expression levels. Here, IDO1 is linked to neoplasm.