GJA1 and cardiac rhythm disease: Regions of heterogeneous Cx43 expression and gap junction conductance have been described in infarcted and failing hearts (Poelzing and Rosenbaum, 2004; Cabo et al., 2006; Akar et al., 2007), can lead to source/sink mismatch and block of propagation (Kleber and Rudy, 2004), and are likely to provide the “substrate” that leads to initiation of cardiac arrhythmias in diseased hearts.