Median levels of QTc prolongation across 24 hours for the Healthy, LQT1, LQT2 and LQT3 cohorts. This plot can be thought of as the chance to detect prolonged QTc (though, because this is the median, 100% indicates that most, not all, beats show QTc prolongation at a given hour). Prolongation in all LQTS cohorts is significantly higher than in healthy subjects (p<10−6 at all times). Availability of Holter data varied from hour to hour, as patients may have been recorded for (e.g.) 22–23 hours rather than the full 24. We therefore report N values as the minimum across all data points.