Fragments of regulatory DNA of cardiac genes drive reporter gene expression in sometimes unexpected subdomains of the heart. These patterns have revealed that the regulatory DNA of genes consists of distinct subfragments (regulatory modules) that are active in different regions of the developing heart. In this review we give an overview of the activity of regulatory modules in vivo. Furthermore, we investigated the relationship between the activity domains of the regulatory modules, the building blocks of the heart and the developmental patterning of the myocardium. Most of the regulatory modules show a domain of activity broader than the morphological boundary of a cardiac compartment and seem to respond to a patterning program along the antero-posterior axis.