KCNE1 and familial long QT syndrome: Here, we innovatively leveraged IBD segments in a large biobank to investigate a rare variant in KCNE1 that is causal for Long QT syndrome (LQTS), a well-recognized, rare cause of syncope and sudden cardiac death (SCD) with an estimated prevalence of 1:2000.8KCNE1 mutations cause Type 5 Long QT Syndrome (LQT5 [MIM: 613695]),9 a subtype accounting for 1–2% of autosomal dominant congenital LQTS cases.