fluorescence-activated cell sorting (facs) is a powerful analytical technique that separates cells according to their content of particular molecules. researchers can use fluorescence-labeled antibodies specific for a cell surface protein to detect cells containing a molecule of interest. suppose that a researcher wants to isolate cells that possess a receptor that detects and binds bacterial degradation products. however, the researcher does not have an antibody directed against this receptor. which fluorescence-labeled molecule should the researcher prepare to identify the cells of interest?