Receptors are crucial for intercellular communication in our body. We develop systems to study effects of thousands of different receptor mutations on receptor function. We use the HEK293 cell line, genetically engineered to allow expression of one single receptor mutant per cell, each coupled to a unique DNA barcode. We generate thousands of cells, each expressing a different mutant. The cells contain a reporter gene that generates a fluorescent signal when an active receptor is stimulated by its ligand. Cells can thus be separated in fractions with  active and  inactive mutants via fluorescence-activated cell sorting.The mutants in the cells are identified and quantified after extraction, amlification and Illumina sequencing of their unique DNA barcodes.This identifies mutants that are likely involved in disease and improves our knowledge of the mechanism of the receptor.