Using a humanized mouse model of melanoma, Somasundaram et al. identified that after PD-1 blockade, mast cells co-localize with T regulatory cells in regions of the tumor with reduced Granzyme B+ CD8+ immune cells and decreased HLA-class I expression, indicating a potential mechanism of resistance to PD-1 blockade [30]. The gene discussed is PDCD1; the disease is melanoma.