Over fifteen SCN5A mutations have been identified in the DIII–DIV linker (Figure 6), leading to either LQTS (e.g. p.G1481E, p.M1498T, p.1500delK, p.L1501V, p.1505–1507delKPQ, p.1507–1509delQKP) [40], [41], [42], [43] due to gain-of-function as a result of residual current [33] or to BrS (e.g. p.Q1476X, p.K1493X, p.Y1494N, p.1500delK, p.L1501V, p.G1502S, p.R1512W, p.I1521K) [23], [44], [45], [46], [47] as a result of reduced availability and/or slowing of recovery from inactivation [33] leading to loss-of-function. The gene discussed is SCN5A; the disease is familial long QT syndrome.