Tumor necrosis factor receptor superfamily, member 4 （TNFRSF4）, also known as CD134 and OX40 receptor, is a member of the TNFR-superfamily of receptors which is not constitutively expressed on resting naive T cells, unlike CD28. OX40 is a secondary co-stimulatory immune checkpoint molecule, expressed after 24 to 72 hours following activation; its ligand, OX40L, is also not expressed on resting antigen presenting cells, but is following their activation. Expression of OX40 is dependent on full activation of the T cell; without CD28, expression of OX40 is delayed and of fourfold lower levels. OX40 binds TRAF2, 3 and 5 as well as PI3K by an unknown mechanism. OX40 has been implicated in the pathologic cytokine storm associated with certain viral infections, including the H5N1 bird flu. An anti-OX40 antibody GSK3174998 has started clinical trials as a cancer treatment.