Calibrator 21 is a lyophilized blend of BAFF-R/TNFRSF13C, CD40L (soluble), FGF (basic), GITR/TNFRSF18, PD-L1, PD-L2, PlGF, RANKL/TNFSF11, and VEGF-D. 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. GITR/TNFRSF18, PD-L1, and PD-L2 were expressed in a human cell line, CD40L (soluble), FGF (basic), and PlGF were expressed in E. coli, RANKL/TNFSF11 was expressed in a hamster cell line, BAFF-R/TNFRSF13C was expressed in a mouse cell line, and VEGF-D was expressed in an insect cell line. Instructions on using Calibrator 21 are provided in U-PLEX Immuno-Oncology Group 1 Human Product Inserts.