(Likely) Pathogenic variants of <i>SCN5A</i>, the gene coding for the alpha subunit of the main cardiac sodium voltage-gated channel, are a known cause of this subset.<h4>Case summary</h4>A 17-year-old male presents with new-onset severe left ventricular systolic dysfunction with atrial flutter and significant ventricular ectopy. The gene discussed is SCN5A; the disease is atrial flutter.