Germline mutations of the TBX5 gene were identified as the primary cause in up to 70 % of patients with Holt-Oram syndrome (HOS), an autosomal dominant disorder characterized by malformations of the upper limbs and cardiac defects. Furthermore, somatic mutations of the TBX5 gene have been described in diseased heart tissues of patients with congenital heart defects of different cause. The relationship between genotype and phenotype remains unclear and the underlying mechanism of the pathogenic effect is not solved. In this report, we introduce the 'TBX5 Gene Mutation Database,' an online locus specific database containing germline and somatic mutations of the TBX5 gene. The permanently updated data collection includes all reported mutations beginning with the first description of the gene in 1997. With our database we complement the existing resources by: 1) giving a complete review of the so far reported mutation spectrum in TBX5 considering the clinical relevance; 2) linkage of the mutational data to the corresponding gene location and PubMedAbstracts; and 3) additional links to other related resources like SNP database, sequences and literature references. The usage of our database will help to quickly find informations about genetic variations within the TBX5 gene. Here we describe the database structure, content, and potential applications (http://www.uni-leipzig.de/∼genetik/TBX5).