How is the specificity defined? We observed that the interaction between Tbx5 and myocardin helps mediate distinct smooth muscle and cardiac gene expression profiles. In differentiating smooth muscle cells, Tbx5 staining showed that it was present only in the cytoplasm, instead of the nucleus. Therefore the expression of cardiac genes in smooth muscle cells won’t be activated by the cooperation of Tbx5 and myocardin; however, the activation of smooth muscle genes by myocardin is not affected. Previous studies suggest that the cellular localization of Tbx5 protein might be controlled by its interaction partners, such as LMP4. It will be important to determine whether subcellular location of the Tbx5 proteins contributes to its function in the control of cardiac and smooth muscle gene expression. Tbx5 belongs to the family of T-box containing transcription factors. Several members of this family of transcription factors are also expressed in the heart. Interestingly, Tbx2 was shown to repress the expression of the ANF gene, in part, by impairing the recruitment of Nkx2.5 to the TBE or the NKE. On the other hand, Tbx20 was reported to synergize with Tbx5 to activate cardiac gene expression. Our data demonstrate that Tbx5 and myocardin synergistically activate the expression of cardiac, but not smooth muscle genes. It will be interesting to test whether other members of the Tbx family of transcription factors will also physically and functionally interact with myocardin to regulate cardiac gene expression during development. Multiple missense mutations of the human Tbx5 gene have been associated with the HOS. Interestingly, these mutations do not uniformly impair the function of Tbx5 in the same manner. For example, Tbx5 mutations G80R and R237Q have different effects on its synergistic transactivation of the cardiac-specific ANF gene, which will Tofacitinib distributor likely translate as subtle differences in the phenotype and severity of the HOS. The Tbx5 G80R mutant displayed more severe defect in co-operating with Nkx2.5 to activate the ANF than that of the R237Q mutant. In addition, the synergy of Tbx5 with Sall4 was slightly reduced by mutations Q49K and T54I but dramatically impaired by mutations Q80R and R237W for FGF10 activation. Given the fact that the expression of Tbx5 downstream genes is significantly affected by the dosage of Tbx5, it is reasonable to speculate that the interaction between Tbx5 and myocardin may differentially affect the expression of some genes, like ANF, but not others like SM22. The molecular mechanisms uncovered in this study suggest that the interaction of transcription factors contribute to the expression of their target genes and human disease. Prion diseases are neurodegenerative diseases characterized by misfolding of prion protein leading to pathologic amyloid deposits in brains of humans and other mammals. Cellular prion protein is composed of an N-terminal unstructured part and a globular Cterminal domain, composed of three a-helices and an antiparallel two-stranded b-sheet. This protein fold, which is conserved in all vertebrate prion proteins with determined structure, is stabilized by a tightly packed hydrophobic core.