A number of alternative parameters such as high baseline levels of Foxp3 and IDO expression54, increased TILs and Th1 cells at baseline56, MDSC numbers50, 57, 58, T cell ICOS expression as pharmacodynamic markers59, and (more recently) high mutational load and neoantigen landscape60, 61, have yet to be prospectively studied as biomarkers for the efficacy of immunotherapy for melanoma. Here, IDO1 is linked to melanoma.