Though MEX may inhibit late INa in LQT3 patients carrying SCN5A mutations, some LQT3 patients with life-threatening arrhythmias were reported to be non-responsive to MEX, and the QTc intervals were not shortened by MEX in some mutations, including M1652R (Ruan et al., 2007). The gene discussed is SCN5A; the disease is long QT syndrome 3.