TSHR cleavage assay. CHO-hTSHR cells were treated with increasing concentrations of TSH for 24 hours and stained with two monoclonal TSHR-Ab’s in order to examine the effect of TSH on TSHR cleavage. RSR-1 (white bars) recognized the β subunit of TSHR, in contrast to the recognition of the region subjected to receptor cleavage (residues 317–366) by RSR-4 (black bars). It should be noted that RSR-4 had no TSH-competing activity. Reduction of immunoreactivity by RSR-4 was a measure of TSHR cleavage and was observed after treatment with TSH (more than 10 μU/ml).