Not a B-cell marker -

Tumor necrosis factor receptor superfamily, member 4 (TNFRSF4), also known as CD134 and OX40, 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 has been implicated in the pathologic cytokine storm associated with ceain viral infections, including the H5N1 bird flu.