Tumor necrosis factor ligand superfamily member 4(TNFSF4/OX40L) is a single-pass type II membrane protein.OX40L is expressed on the surface of activated B cells, T cells, dendritic cells and endothelial cells. OX40L binds to OX40 (CD134), a member of the TNF receptor superfamily that is expressed predominantly on activated CD4+ T cells. OX40-OX40L co-stimulates signal to promote the survival and proliferation of activated CD4+ T cells and prolong the immune response. It involved in T-cell proliferation and cytokine production. Additional, it has been found association with systemic lupus erythematosus, no association with occurrence of atherosclerosis.

Tumor necrosis factor ligand superfamily member 4 (TNFSF4) is also known as glycoprotein Gp34, OX40 ligand (OX40L),which belongs to the tumor necrosis factor family. It is expressed on such cells as DC2s (a subtype of dendritic cells) enabling amplification of Th2 cell differentiation.