Holt-Oram syndrome (HOS), which results from mutations in the gene TBX5, causes cardiac rhythm abnormalities. Moskowitz et al.(p. 4107) now report several distinct roles for Tbx5 in the patterning and maturation of the mouse heart conduction system – the myocardial cells that coordinate the contraction of the multichambered heart. They show that Tbx5 is expressed throughout the central conduction system from early heart development. Mice with Tbx5 haploinsufficiency have postnatal morphological and functional defects in the proximal atrioventricular conduction system and earlier patterning abnormalities in the distal atrioventricular conduction system. In particular, the complete absence of a part of the conduction system – the right bundle branch – results in a conduction delay. Tbx5 is the first gene implicated in both the patterning and maturation of the central conduction system, and Moskowitz et al. consider how this may shed light on the pathology of HOS.