A C981F mutation in Nav1.5 was reported in a patient with long QT syndrome (Kapplinger et al., 2009; Pei et al., 2016), which corresponds to an intracellular loop cysteine on the domain II-III linker region (Figure 2), a location ideally situated to regulate pore activity. The gene discussed is SCN5A; the disease is Prolonged QT interval.