Zebrafish Mef2ca and Mef2cb are essential for both first and second heart field cardiomyocyte differentiation

► Mef2c homologues are essential for heart formation in zebrafish. ► Mef2ca and Mef2cb function redundantly to drive differentiation of FHF and SHF CMs. ► mef2ca and mef2cb single mutants have a seemingly normal heart. ► Mef2cb overexpression during early embryonic development converts cells into skeletal muscle.