Typical dose-response curves of cAMP accumulation induced by TSH on HEK-293 cells transfected with pcDNA3.1 plasmids encoding WT or mutated TSHRs. In (a), the cells were transfected with the same amount of TSHR coding plasmids, leading to the cell membrane expression shown in insert. In (b), the cells were transfected with different amounts of TSHR coding plasmids to obtain similar levels of cell surface expression, shown in insert. The total amount of pcDNA3.1 plasmids was kept constant by addition of empty plasmids. Receptor symbols: closed circles, WT; closed down triangles, L2.58P; open squares, L2.59P; open triangles, I2.60P; closed diamonds, A5.50P; closed squares, L2.58PA5.50P; closed triangles, P2.59PA5.50P; open diamonds, I2.60PA5.50P.