GPRC5D is selectively expressed on the surface of MM cells. (A-B) A bioinformatic analysis of 2 NCI public data sets shows upregulation of GPRC5D mRNA expression on MM patient CD138⁺ cells as well as SMM and plasma-cell leukemia samples (PCL). mRNA expression was plotted on log2 scale on the y-axis. (C) GPRC5D cell-surface protein expression on MM cells (AMO-1, EJM, H929, MM.1R, MOLP-8, and OPM2), B cells (Daudi), and myeloid cells (KG-1 and MOLM-13) was labeled using an anti-GPRC5D antibody (red line; 571961 clone; R&D Systems) or an isotype control antibody (gray line; IC0041A clone; R&D Systems) for 30 minutes at 4°C and analyzed by flow cytometry. (D) Images of cells with representative distribution of GPRC5D staining on the cell surface are shown. Merged images of GPRC5D staining and bright field are used to evaluate the localization of protein expression on the cell surface. (E) GPRC5D receptor density quantitation of CD138⁺ BM MNCs from healthy (n = 21) and MM patient samples (n = 23) shows averages of 1600 to 1800 receptors/cell (QuantiBRITE kit; difference is not statistically significant [N.S.]). (F) GPRC5D mRNA levels in various human tissues relative to H929 cells. Samples were analyzed for GPRC5D and glyceraldehyde 3-phosphate dehydrogenase mRNA expression using a TaqMan probe, and relative levels were plotted on the y-axis. Elevated levels of GPRC5D mRNA expression were observed in lymph node and spleen with minimal levels in lung, skin, and testis. The majority of the tissues were negative for GPRC5D. MGUS, monoclonal gammopathy of undetermined significance. GSE, Genomic Spatial Event database.