The human SCN5A gene encodes the pore-forming subunit of Nav1.5, a voltage-gated sodium channel expressed in heart and upregulated in breast cancer cells. Mutations in SCN5A are responsible for some forms of long QT and short QT syndromes, progressive cardiac conduction defect, sick sinus node syndrome, and dilated cardiomyopathy, Brugada syndrome, congenital heart block, Nav1.5 channels are anti-targets in cardiac risk assessment.