Using single-cell suspensions prepared from malignant melanoma resections banked at MSK and classified as low or high based on their intracellular expression of IDO (Fig. 1b), we detected higher levels of immune-regulatory factors (IL-10, PD-L1, and CTLA-4) in IDOhigh samples compared to IDOlow samples (Fig. 1c). Here, CTLA4 is linked to melanoma.