(A) Baseline action potential duration at 90% repolarization (APD90) was similar among C1-SS (n=27), A1-SS (n=21), C1-SY (n=39), A1-SY (n=19), and C1-YY (n=15) cells. (B) Representative traces during dofetilide perfusion in iPSC-CMs with the SS (C1-SS) and YY (C1-YY) genotype. Traces include APD90 at baseline, 10, and 30 nM dofetilide perfusion. The trace of C1-YY at 30 nM dofetilide was paced at 0.25 Hz due to extreme APD prolongation (#). (C) Dofetilide concentration that prolonged APD90 by 100 ms ([Dof]Δ100ms). (D) Absolute change in APD90 at 10 nM dofetilide. Open symbols denote data obtained from cells with the SS genotype, grey symbols from the SY genotype, and black symbols the YY genotype. Statistical comparisons were between lines, and between genotypes by combining same genotypes.*p<0.05, **p<0.01, †p<0.001.