Rationale: Mutations in the SCN5A gene, encoding the alpha subunit of the Nav1.5 channel,
cause a life-threatening form of cardiac arrhythmia, long QT syndrome type 3 (LQT3).
Mexiletine, which is structurally related to the Na+ channel-blocking anesthetic lidocaine,
is used to treat LQT3 patients. However, the patient response is variable, depending
on the genetic mutation in SCN5A.