Mutations in the TBX5 transcription factor gene cause human cardiac malformation in Holt-Oram syndrome. To identify and localize TBX5 during cardiac morphogenesis, we performed immunohistochemical studies of TBX5 protein cardiac expression during human embryogenesis. Specific antibody to human TBX5 was generated in rabbits with a TBX5 synthetic peptide and affinity purification of antiserum. Anti-TBX5 was used in immunohistochemical analyses of human cardiac tissue. In embryonic and adult heart, TBX5 is expressed throughout the epicardium and in cardiomyocyte nuclei in myocardiurn of all four cardiac chambers. Endocardial expression of TBX5 is only present in left ventricle. Asymmetric left-sided transmyocardial gradients of TBX5 protein expression were observed in embryonic but not adult hearts. Human cardiac expression of TBX5 protein correlates with the cardiac manifestations of Holt-Oram syndrome. TBX5 transmyocardial protein gradients may contribute to normal patreining of the human heart during embryogenesis. (C) 2000 Wiley-Liss, Inc.