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. The DR4 and DR5 receptors are both produced as type I transmembrane proteins, which contain an extracellular domain, a transmembrane domain, and a cytoplasmic domain. The recombinant soluble forms of DR4 and DR5 consist of the TNFR-homologous, cysteine-rich portion of their respective extracellular domains. 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.