A seminal work by Ayers et al. introduced a T cell-inflamed gene expression profile (GEP) centered on IFN-γ-responsive genes (e.g., CXCL9, CXCL10, IDO1, STAT1, GZMB) that correlated with response to anti-PD-1 therapy across multiple tumor types, including melanoma [99]. Here, IDO1 is linked to melanoma.