Congenital heart disease is a significant cause of morbidity and mortality in humans, and gene mutations that underlie some of these anomalies are now being described. The NKX2.5 gene, which encodes a homeobox transcription factor, was initially discovered in mice through its similarity to the tinman gene of the fruitfly Drosophila. Tinman is required for formation of the dorsal pulsatile vessel or 'heart' of the fly. Tinman and NKX2.5 share structural and functional features, and in mice the gene is required for normal cardiac looping and differentiation of chamber myocardium. Humans with heterozygous mutations in the NKX2.5 gene generally have a disorder involving progressive atrio-ventricular conduction block and atrial septal defect, although sometimes other abnormalities including tetralogy of Fallot. The TBX5 gene, which encodes another cardiac transcription factor that collaborates with NKX2.5, is also an important cardiac disease gene, with heterozygous mutations responsible for Holt-Oram (hand/heart) syndrome. These contributions to human pathology underscore the relevance of studying biological phenomena in lower organisms, and examination of other genes acting in this and associated pathways will expand our knowledge of congenital abnormalities and disease predisposition, and improve genetic counseling.