dHAND and eHAND are related basic helix-loop-helix (bHLH) transcription factors that are expressed in mesodermal and neural crest-derived structures of the developing heart. In contrast to their homogeneous expression during avian cardiogenesis, during mouse heart development we show that dHAND and eHAND are expressed in a complementary fashion and are restricted to segments of the heart tube fated to form the right and left ventricles, respectively. dHAND and eHAND represent the earliest cardiac chamber-specific transcription factors yet identified. Targeted gene deletion of dHAND in mouse embryos resulted in embryonic lethality at embryonic day 10.5 from heart failure. Our description of the cardiac phenotype of dHAND mutant embryos is the first demonstration of a single gene controlling the formation of the mesodermally derived right ventricle and the neural crest-derived aortic arches and reveals a novel cardiogenic subprogramme for right ventricular development.
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