The protein encoded by this gene is a cytokine that belongs to the tumor necrosis factor (TNF) ligand family. This cytokine is a ligand for receptor TNFRSF4/OX4. It is found to be involved in T cell antigen-presenting cell (APC) interactions. In surface Ig- and CD40-stimulated B cells, this cytokine along with CD70 has been shown to provide CD28-independent costimulatory signals to T cells. This protein and its receptor are reported to directly mediate adhesion of activated T cells to vascular endothelial cells.