Jervell and Lange-Nielsen syndrome (MIM 220400; JLNS), is a rare form of 
profound congenital deafness combined with syncopal attacks and sudden death due 
to prolonged QTc; it is an autosomal recessive trait. After its first 
description in Norway in 1957, later reports from many other countries have 
confirmed its occurrence. Nowhere is the prevalence so high as in Norway, where 
we estimate a prevalence of at least 1:200,000. The KCNQ1 and KCNE1 proteins 
coassemble in a potassium channel, and mutations in either the KCNQ1 gene or the 
KCNE1 gene disrupt endolymph production in the stria vascularis in the cochlea, 
causing deafness. KCNQ1 seems to be the major gene in JLNS. Long QT syndrome 
(LQTS) is a separate disorder of either autosomal dominant or recessive 
inheritance caused by mutations in four different ion channel genes; KCNQ1 is 
the one most frequently involved. Some heterozygous carriers of JLNS mutations 
in either gene may suffer from prolonged QTc and be symptomatic LQTS patients 
with a need for appropriate medical treatment to prevent life-threatening 
cardiac arrhythmia. In general, frameshift/stop mutations cause JLNS, and 
missense/splice site mutations cause LQTS, but a precise genotype-phenotype 
correlation in LQTS and JLNS is not established, which complicates both genetic 
counseling and clinical risk evaluation in carriers. We review JLNS from a 
Norwegian perspective because of the unusually high prevalence, the genetic 
homogeneity associated with considerable mutational heterogeneity, and some 
evidence for recurrent mutational events as well as one founder mutation. We 
outline the clinical implications for investigation of deaf children and cases 
of sudden infant death syndrome as well as careful electrocardiographic 
monitoring of identified mutation carriers to prevent sudden death. Am. J. Med. 
Genet. (Semin. Med. Genet.) 89:137-146, 1999.