Recombinant human CD137 (secreted), encompassing amino acids 24-186. The protein is fused at the C-terminus with the Fc fragment of human IgG1 followed by an Avi-Tag™. The recombinant protein was enzymatically biotinylated using the Avi-Tag™ and affinity purified. HiP™ indicates a high purity protein (≥90% pure) and less than 10% aggregation as measured by gel filtration.