A process for testing a tumor sample for the presence or absence of a PD-1:PD-Ligand proximity biomarker that is predictive of an anti-tumor response to treatment with a PD-1 antagonist, which comprises: (a) obtaining a PD-1:PD-Ligand proximity score for the tumor sample by the process of; obtaining, by imaging, an image of tissue that has been removed from the tumor sample and immunohistochemically stained for PD-1 and PD-Ligand expression in a manner that allows stained PD-1 cells to be distinguished from stained PD-Ligand cells; defining in the image one or more regions of interest (ROIs) that comprises neoplastic cells and associated stroma, wherein substantially all of the neoplastic cells and associated stroma in the image are contained in the defined ROIs; randomly creating across each defined ROI a plurality of subregions of substantially the same shape and size, wherein each of the subregions defines an area that is large enough to include a spatially proximal pair of a stained PD-1 cell and a stained PD-Ligand cell and small enough to exclude pairs of stained PD-1 and PD-Ligand cells that are not spatially proximal; and calculating the percent of all of the subregions that are positive for both stained PD-1 cells and stained PD-Ligand cells to generate the PD-1:PD-Ligand proximity score for the tumor sample; (b) comparing the PD-I:PD-Ligand proximity score for the tumor sample with a threshold PD-1:PD-Ligand proximity score; (c) classifying the tumor as biomarker positive or biomarker negative, wherein if the proximity score for the tumor sample is equal to or greater than the threshold proximity score, then the tumor is classified as positive for the PD-I:PD-Ligand proximity biomarker, and if the obtained score is less than the threshold score, then the tumor is classified as negative for the PD-1:PD-Ligand proximity biomarker; and (d) using the classification to predict if a subject is likely to respond to a PD-1 antagonist wherein the PD-1 antagonist is a monoclonal antibody that inhibits the binding of PD-L1 to PD-1, or the PD-1 antagonist is a monoclonal antibody that inhibits the binding of PD-L1 and PD-L2 to PD-1, or the PD-1 antagonist is MK-3475 or nivolumab. (a) obtaining a PD-1:PD-Ligand proximity score for the tumor sample by the process of; obtaining, by imaging, an image of tissue that has been removed from the tumor sample and immunohistochemically stained for PD-1 and PD-Ligand expression in a manner that allows stained PD-1 cells to be distinguished from stained PD-Ligand cells; defining in the image one or more regions of interest (ROIs) that comprises neoplastic cells and associated stroma, wherein substantially all of the neoplastic cells and associated stroma in the image are contained in the defined ROIs; randomly creating across each defined ROI a plurality of subregions of substantially the same shape and size, wherein each of the subregions defines an area that is large enough to include a spatially proximal pair of a stained PD-1 cell and a stained PD-Ligand cell and small enough to exclude pairs of stained PD-1 and PD-Ligand cells that are not spatially proximal; and calculating the percent of all of the subregions that are positive for both stained PD-1 cells and stained PD-Ligand cells to generate the PD-1:PD-Ligand proximity score for the tumor sample; (b) comparing the PD-I:PD-Ligand proximity score for the tumor sample with a threshold PD-1:PD-Ligand proximity score; (c) classifying the tumor as biomarker positive or biomarker negative, wherein if the proximity score for the tumor sample is equal to or greater than the threshold proximity score, then the tumor is classified as positive for the PD-I:PD-Ligand proximity biomarker, and if the obtained score is less than the threshold score, then the tumor is classified as negative for the PD-1:PD-Ligand proximity biomarker; and (d) using the classification to predict if a subject is likely to respond to a PD-1 antagonist wherein the PD-1 antagonist is a monoclonal antibody that inhibits the binding of PD-L1 to PD-1, or the PD-1 antagonist is a monoclonal antibody that inhibits the binding of PD-L1 and PD-L2 to PD-1, or the PD-1 antagonist is MK-3475 or nivolumab.