Differential regulation of cardiac gene expression in vertebrates has been extensively documented in the context of atrial and ventricular morphogenesis. Recent data, largely from the analysis of transgene and endogenous gene expression patterns, have revealed transcriptional differences between left and right cardiac chambers which suggest that the heart is composed of a series of distinct transcriptional domains. Such phenomena provide regional markers (cardiosensors) for the fine analysis of normal and abnormal heart development. Regional subdivisions and transcriptional heterogeneity within the myocardium emerge in response to patterning of the precardiac mesoderm and early heart tube on the anterior-posterior axis, and to embryonic left-right signals which dictate the direction of cardiac looping. Several families of transcription factors have been characterized which may be implicated in the regionalization of myocardial gene expression.