Genes such as KCNQ1 and SCN5A (associated with congenital long QT syndrome) have been described in Mongolia and the relative genetic isolation has been proposed as a possible cause for QT prolongation in this population.25–29 Single nucleotide polymorphisms in the UGT1A gene are known to affect the metabolism of MFX.30 The absence of increased risk in the Vietnamese patients in STREAM Stage 1 would support regional variation among Asian populations. Here, KCNQ1 is linked to familial long QT syndrome.