High-throughput screening for a GPCR for heme in antagonist mode. (A) Schematic overview of the PathHunter β-arrestin cell–based assay in agonist (left) and antagonist (right) mode. (B-D) Percentage of reporter activity as compared to control vehicle of 168 GPCRs tested in the GPCR Orphan and MAX and for 3-μM heme in agonist mode. (E-F) Percentage inhibition of reporter activity induced by 3-μM heme in the presence of the cognate ligand, compared to the cognate ligand with vehicle, tested in the GPCR MAX panels in antagonist mode. Figure created with BioRender.com.