We used a pedigree-based measured genotype association analysis to study the association between QTc prolongation and three previously published NOS1AP sequence variants, chosen for their strong association with QTc in LQTS and normal populations, in two large Swedish LQT1 founder populations, revealing a significant association for rs12143842 and rs16847548, but not rs4657139. This evidence concerns the gene NOS1AP and familial long QT syndrome.