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. The gene discussed is UGT1A1; the disease is familial long QT syndrome.