The 3�� end of the PromoterLong fragment, encompassing the PromoterShort fragment, contains core promoter elements and is particularly rich in consensus binding sites for transcription factors implicated in early heart development and/or embryogenesis. The AHR-ARNT (aryl hydrocarbon receptor/AhR nuclear translocator) heterodimers are coexpressed in the developing chick heart and potentially have role in cardiac development and growth [14, 15]. The AP4 (activator protein 4) is possibly involved in the regulation of the ��-myosin heavy chain gene in rat cardiomyocytes, and H1F-1 (hypoxia induced factor 1 (HIF-1)) has been shown to be essential for normal cardiac development [16, 17]. AP2 (activator protein 2) is developmentally regulated, and cardiac neural crest cells expressing AP2 have been shown to be important for cardiac looping in zebrafish [12, 18]. The NF1 (nuclear factor 1) is an important regulator in the RAS signal transduction pathway and for normal embryogenesis and normal heart development [19, 20]. As mentioned above, E2F has been shown to be implicated in the control of cardiac myocyte growth and is a core promoter element [8]. The two core promoter elements, ZF5 (found in 50.75% of human core promoters) and E2F (found in 74.25% of human core promoters) [8], are represented in the PromoterShort fragment. In addition, this region is GC-rich, which is common for core promoters [21]. These combined features suggest that the PromoterShort fragment contains or is the core promoter of RhoA. This conclusion is also supported by the results obtained from the luciferase reporter assays, which indicate that PromoterShort has less promoter activity than PromoterLong. The increased promoter activity for PromoterLong indicates that elements upstream of PromoterShort are necessary to enhance the promoter activity, suggesting that this region is the proximal promoter of RhoA. The increased activity of the RhoA PromoterShort fragment in differentiating cardiomyocytes compared to nondifferentiated cells indicates that RhoA has an important role in differentiating cardiomyocytes and suggests that some of the transcription factors expressed in differentiating P19CL6 cells bind to the RhoA promoter to increase transcriptional activity. These results support earlier findings, which implicate RhoA as an important factor in early heart development and normal embryogenesis. The putative human RhoA promoter was previously cloned to investigate the PKG-(cGMP-dependent-protein kinase-) dependent regulation of RhoA in arterial smooth muscle cells [22]. Two different length fragments (913bp and 118bp) just upstream of the ATG codon were assayed for luciferase activity, and, interestingly, the shorter fragment exhibited the same promoter activity as the longer fragment.