TRAIL Receptor-1/DR4 and TRAIL Receptor-2/DR5 belong to the TNFR superfamily of transmembrane proteins and contain a cytoplasmic death domain, which can activate the cell's apoptotic machinery. These receptors are activated by binding to either membrane anchored or soluble TRAIL/Apo2L. Recombinant human soluble TRAIL Receptor-1/DR4 is a 22.7 kDa protein (215 amino acid residues) consisting of the TNFR homologous, cysteine rich portion of the extracellular domain.