SCN5A and ventricular fibrillation: This mutation causes dysfunction of the alpha-subunit of the cardiac Na+-channel, Nav1.5; the mutant form of Nav1.5-R104W can interact with WT channels, causing a decrease in Na+ current in the sarcoplasmic reticulum of cardiomyocytes, which is responsible for the development of ventricular fibrillation followed by possible sudden cardiac death.