A recent study in colorectal and lung cancer has shown that CD39 accurately distinguishes between tumour-specific (PD-1+CD39+) and bystander (PD-1+CD39−) TILs.27 In another study, co-expression of PD-1 with markers of tissue-resident memory cells, such as CD39 and CD103, seemed to identify tumour-specific TILs.29 Presumably, combinations of PD-1 with other markers would be necessary to accurately identify the full repertoire of tumour-specific TILs, especially in tumours in which these cells are rare. The gene discussed is PDCD1; the disease is neoplasm.