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Objective: SCN5A encodes alpha subunit of the major sodium channel (Nav1.5) in human cardiac tissue. Malfunction of this cardiac sodium channel is associated with a variety of cardiac arrhythmias and myocardial inherited diseases. Methods: Fifty-three members from three families each diagnosed with long-QT syndrome type 3 (LQTS3), Brugada syndrome (BrS), or sick sinus syndrome (SSS) were included in this observational, cross-sectional study. In this study, we analyzed the sequences of coding region of the SCN5A gene. Results: Eleven members of the LQTS family (39%) showed p.Gln1507-Lys1508-Pro1509del mutation, 8 of BrS family (50%) showed p.Arg222Ter nonsense mutation, and 5 of 9 SSS family members (55%) showed a novel p.Met1498Arg mutation in the SCN5A gene. Conclusion: p.Gln1507-Lys1508-Pro1509del mutation, p.Arg222Ter nonsense mutation, and p.Met1498Arg in LQTS, BrS, and SSS, respectively, are reported for the first time in the Iranian population. Information regarding underlying genetic defects would be necessary for verifying certain clinically diagnosed arrhythmia types, carrier screening in affected families, and more precise therapy of the patients are required. [ABSTRACT FROM AUTHOR]