The familial long-QT syndrome (LQTS) is an infrequently occurring disorder in which affected family members have QT prolongation on the ECG, often associated with recurrent syncope and fatal ventricular arrhythmias. Autosomal recessive and autosomal dominant modes of inheritance were suggested by the pattern of occurrence of this disorder in the first reported LQTS families. Statistical genetic analysis (segregation analysis) has substantiated a major gene effect on QTc length in two large pedigrees. Gene linkage studies have uncovered tight linkage between a DNA marker at the Harvey ras-1 locus on chromosome 11 and LQTS in one large pedigree, substantiating a genetic basis of this disorder.