This report describes a three-generation family with a severe phenotype of 
long-QT syndrome-1 (LQTS-1) caused by a single nucleotide mutation in the 
KQT-like, voltage-gated potassium channel-1 gene (KCNQ1; MIM 607542). Two 
members of the family died suddenly in their childhood, and all eight surviving 
members with prolonged QT have a heterozygous missense mutation resulting in a 
glycine-to-glutamate amino acid substitution at position 316 of the potassium 
channel. In this family, the newly reported mutation, guanine-to-adenosine at 
position 947 in the KCNQ1 gene, exhibits a dominant trait of LQTS with complete 
penetrance, in contrast to the relatively reduced clinical penetrance found in 
most LQTS cases.