An Nkx2.5Cre/+ knock-in mouse line was kindly provided by Dr. Robert Schwartz14 (link). An αMHCCre/+ transgenic mouse line was kindly provided by Dr. E. Dale Abel15 (link). ROSA26eGFP-DTA and ROSA26LacZ mouse lines were purchased from the Jackson Laboratory16 (link), 17 (link). Experimental animal protocols were approved by the Institutional Animal Care and Use Committees of Massachusetts General Hospital and Stanford University. All experiments were performed on somite-matched embryos or sex-matched adult mice.