,2,3 In colorectal cancer (CRC), tumor-infiltrating myeloid cells, in particular M2 macrophages, have been associated with poor prognosis,4 and only patients who have a mismatch repair-deficient (MMRd) microsatellite instability (MSI) phenotype show partial or complete responses to anti-PD-1 immune checkpoint inhibition.5 The gene discussed is PDCD1; the disease is neoplasm.