Mutations in the SCN5A gene (LQTS type 3), which encodes the main protein of INa, and mutations in the genes encoding its regulatory proteins (LQTS 4, LQTS 9, LQTS 10 and LQTS 12) can in fact cause LQTS as long as the mutations result in a gain-of-function of the late sodium current. This evidence concerns the gene SCN5A and familial long QT syndrome.