An expanded Materials and Methods section is available in the online data supplement at http://circres.ahajournals.org and describes the following: inducible Nkx2-5 knockout mice; Southern, Northern, and Western blotting and immunostaining; surface ECG recording and ultrasound imaging; histological analyses; gene expression profiling; measurement of sodium current and simultaneous recording of cardiac contraction and Ca2+; real-time RT-PCR; reporter assays; and statistical analyses.