Previously unknown mutations of the KCNH2, SCN5A and KCNQ1 genes are disclosed which are involved in ion channel disruptions associated with short QT syndrome, long QT syndrome, Brugada syndrome and progressive conduction disease. These mutations are utilized to diagnose and screen for short QT syndrome, long QT syndrome, Brugada syndrome and progressive conduction disease, thus providing modalities for diagnosing sudden cardiac death and/or predicting susceptibility to sudden cardiac death. Nucleic acid probes are provided which selectively hybridize to the mutant nucleic acids described herein. Antibodies are provided which selectively bind to the mutant proteins described herein. The mutations described herein are also utilized to screen for compounds useful in treating the symptoms manifest by such mutations.