Counter assays typically follow concentration-response assays to confirm the biological activity of chemical entities identified in the primary assays. Counter assays eliminate active compounds that do not act on the intended target. They are performed using related targets, e.g. a receptor isoform, a homologous gene, etc which are functionally distinct from the intended target. Counter assays are also performed to eliminate cytotoxic compounds. The actives from the primary assay are tested on cultured cells and the cell death is estimated commonly by measuring ATP concentration. Other example for a counter assays a FRET assay when the primary assay used absorption readout; luminescence readout when the primary assay measured fluorescence intensity, etc.