Two knock in (KI) models of SCN5A mutants have been developed (Sendfeld et al., 2019): one in mice, exhibiting an overlap syndrome whose conduction-defect severity was strain-dependent (Remme et al., 2006), and one in pig, showing prolonged P and QRS wave duration and prolonged PR intervals, consistent with slowed cardiac conduction (Park et al., 2015). Here, SCN5A is linked to overlapping connective tissue disease.