['Loss-of-function mutations in the gene KCNQ1 encoding the Kv7.1 K(+) channel cause long QT syndrome type 1 (LQT1), whereas gain-of-function mutations are associated with short QT syndrome as well as familial atrial fibrillation (FAF). However, KCNQ1 mutation pleiotropy, which is capable of expressing both LQT1 and FAF, has not been demonstrated for a discrete KCNQ1 mutation. The genotype-phenotype relationship for a family with FAF suggests a possible association with the LQT1 p.Arg231Cys-KCNQ1 (R231C-Q1) mutation.', 'The purpose of this study was to determine whether R231C-Q1 also can be linked to FAF.', 'The R231C-Q1 proband with AF underwent genetic testing for possible mutations in 10 other AF-linked genes plus KCNH2 and SCN5A. Sixteen members from five other R231C-positive LQT1 families were genetically tested for 21 single nucleotide polymorphisms (SNPs) to determine if the FAF family had discriminatory SNPs associated with AF. R231C-Q1 was expressed with KCNE1 (E1) in HEK293 cells, and Q1E1 currents (I(Q1E1)) were analyzed using the whole-cell patch-clamp technique.', 'Genetic analyses revealed no additional mutations or discriminatory SNPs. Cells expressing WT-Q1 and R231C-Q1 exhibited some constitutively active I(Q1E1) and smaller maximal I(Q1E1) compared to cells expressing WT-Q1.'] Does r231C mutation in KCNQ1 cause long QT syndrome type 1 and familial atrial fibrillation?