Given that KCNQ1 gene mutations account for up to 48% of congenital LQT syndromes (LQT1, OMIM: #192500) and that our functional analyses confirm a deleterious functional effect of the novel KCNQ1dup12 mutation, our sequence and linkage analyses provide striking evidence that the mutation plays a causative role for the LQT trait in this family. The gene discussed is KCNQ1; the disease is long QT syndrome 1.