As an example, it is thought that beta-blockers are less effective in long-QT syndrome 3, which is caused by gain-of-function pathogenic variants in SCN5A (sodium channel protein type 5 subunit alpha, NaV1.5), compared to other forms of long-QT syndrome, and mexiletine may be added in such cases [13••, 26]. The gene discussed is SCN5A; the disease is familial long QT syndrome.