Mutations in the KCNQ1, HERG, SCN5A, minK and MiRP1 genes cause long QT syndrome 
(LQTS), of which there are two forms: the Romano Ward syndrome and the Jervell 
and Lange-Nielsen syndrome. We have performed DNA sequencing of the 
LQTS-associated genes in 169 unrelated patients referred for genetic testing 
with respect to Romano Ward syndrome and in 13 unrelated patients referred for 
genetic testing with respect to Jervell and Lange-Nielsen syndrome. A total of 
37 different mutations in the 5 genes, of which 20 were novel, were identified. 
Among patients with the most stringent clinical criteria of Romano Ward 
syndrome, a mutation was identified in 71%. Twelve of the 13 unrelated patients 
referred for genetic testing with respect to Jervell and Lange-Nielsen syndrome 
were provided with a molecular genetic diagnosis. Cascade genetic screening of 
505 relatives of index patients with molecularly defined LQTS identified 251 
mutation carriers. The observed penetrance was 41%. Although caution must be 
exerted, the prevalence of heterozygotes for mutations in the LQTS-associated 
genes in Norway could be in the range 1/100-1/300, based on the prevalence of 
patients with Jervell and Lange-Nielsen syndrome.