Calibrator 31 is a lyophilized blend of 4-1BBL/TNFSF9, CD40/TNFRSF5, FasL, HAVCR1/KIM-1, and TACI/TNFRSF13B. It is intended for use as an assay calibrator for MSD U-PLEX human assays that measure any of the included proteins. The utility of this product has not been evaluated in other applications. Individual analyte concentrations are provided in the lot-specific certificates of analysis. 4-1BBL/TNFSF9 is a recombinant protein expressed in E. coli; CD40/TNFRSF5 is a recombinant protein expressed in a human cell line; FasL is a recombinant protein expressed in a hamster cell line; HAVCR1/KIM-1 and TACI/TNFRSF13 are recombinant proteins expressed in a mouse cell line. Instructions on using Calibrator 31 are provided in U-PLEX Immuno-Oncology Group 1 Human Product Inserts.