Uncovering a pathogenic variant, in one of the genes known to be associated with LQTS (e.g. KCNH2 gene encoding hERG channel), allows identifying the relatives at risk, in the proband’s family (Alders et al., 1993) (Figure 1A), even when ECGs are not always strongly evocative of the pathology (cf. patient III-1 and patient III-4 in Figure 1B). Here, KCNH2 is linked to familial long QT syndrome.