PDCD1 and cancer: We detected PD1 expression on both TILs and cancer cells in esophageal tumor biospecimens, paralleling findings in human malignant melanoma.7,16,23 Consistent with immunohistochemical studies of normal and malignant hematopoietic and other tissues, PD1 immunoreactivity marked subsets of predominantly small TILs and larger cancer cells that exhibited both cell surface and cytoplasmic staining for PD1, with TILs showing the strongest staining intensity.16,23–26