Conclusion
The Q1491H and G1481V mutations are both located near the IFM motif, which is involved in fast inactivation. These mutations changed the inactivation state, shifting it to a more positive voltage. They also increased the window current, which can result in heart malfunctions such as LQT3. Full biophysical characterization constitutes a strong benefit, providing useful supplemental criterion in the variant classification procedure. These two infants may have benefited from class 1 antiarrhythmic drug therapy, but the lack of effect these drugs is not well understood and warrants further study.