TBX5 mutations cause the cardiac and limb defects of the autosomal dominant Holt-Oram syndrome (HOS). We have explored the role of the TBX5 transcription factor during cardiogenesis and have elucidated some of its functions in regulating myocardial cell proliferation and proepicardial cell migration. Our identification of TBX5 mutations has enabled us to offer genetic testing for diagnosis of HOS in patients and also to perform preimplantation genetic diagnosis on blastocysts for couples desiring to have a child unaffected by HOS. We hope that our genetic testing approach will serve as a paradigm for mutation screening in other inherited syndromes. © 2006 Wiley-Liss, Inc.