The present invention relates to methods for assessing the risk of a patient for developing a potentially fatal cardiac dysrhythmia and for diagnosing Andersen's Syndrome. A tissue sample from a patient is obtained and the DNA or proteins of the sample isolated. From the DNA and protein isolates the sequence of the KCNJ2 gene or the Kir2.1 polypeptide can be obtained. The KCNJ2 gene or the Kir2.1 can be screened for alteration as compared to the wile-type sequence. An alteration in a copy of the KCNJ2 gene or a Kir2.1 polypeptide indicates that the patient has a high risk for developing a cardiac dysrhythmia and can be diagnosed with Andersen's Syndrome. The invention also related to isolated nucleic acid molecules with one or more alterations as compared to the wild-type sequence.