OX40 is expressed constitutively by a subset of Treg cells, but also on activated non‐Treg cells.113 In cancer, Treg cells can comprise a significant proportion of tumour‐resident OX40+ cell types.114, 115, 116 Although OX40‐agonistic reagents are used to stimulate anti‐tumour T‐cell responses, the effect on Treg cells in cancer is not well understood. This evidence concerns the gene TNFRSF4 and cancer.