INA and myotonic dystrophy type 1: Previous studies have also reported that the SCN5A neonatal isoform is overexpressed in DM1 heart samples, including samples from humans13,37.In terms of SCN5A, which codes for NaV1.5, the cardiac Na+ channel, the increase in the neonatal isoform was consistent with the gating changes observed in INa in DM1-1300 cardiomyocytes.