TRAIL/Apo2L is a cytotoxic protein, which activates rapid apoptosis in tumor cells, but not in normal cells. TRAIL induced apoptosis is achieved through binding to two death-signaling receptors, DR4 and DR5. These receptors belong to the TNFR superfamily of transmembrane proteins and contain a cytoplasmic death domain, which activates the cell's apoptotic machinery. Recombinant human soluble TRAIL/Apo2L is a 168 amino acid polypeptide (19.6 kDa), consisting of the TNF homologous portion of the extracellular domain of the full length TRAIL/Apo2L protein.