PCNA and neoplasm: To address this point, we performed co‐immunostaining for PCNA (marker of cell proliferation [39]), pan‐cytokeratin (Pan‐CK, highlighting tumor cells), and DAPI (for nuclei) in N = 2 POSTN‐high and N = 2 POSTN‐low PDX tumors and quantified PCNA expression in stromal and tumoral cells.