Knowing in your heart what's right.

During heart formation in all vertebrate species, the linear heart tube undergoes rightward looping followed by the formation of the atrial and ventricular chambers. The direction of cardiac looping is determined in part by the asymmetric expression of members of the transforming growth factor beta family across the left-right axis of the embryo. The basic helix-loop-helix (bHLH) transcription factors dHAND and eHAND are expressed in the heart tube within specific cardiogenic precursors destined to form the right and left ventricular regions, respectively, and loss-of-function experiments have demonstrated the importance of these factors for looping morphogenesis and ventricular development. We propose a model in which the HAND gene products interpret asymmetric positional information in the developing heart and participate in transcriptional programmes that control development of the right and left ventricular compartments of the heart.