I<sub>Kr</sub> and action potentials were measured using the patch-clamp technique in guinea pig ventricular cardiomyocytes treated with IgG isolated from the sera of an anti-Ro/SSA-52kD antibody-positive patient with LQTS and Torsades de Pointes.<h4>Results</h4>Guinea pigs immunized with Ro/SSA-52kD antigen exhibit QTc prolongation and hERG-cross-reactive anti-Ro/SSA-52kD serum antibodies. Here, KCNH2 is linked to familial long QT syndrome.