Congenital heart defects still constitute serious medical problems. The background of such defects, however, is poorly understood. Little is also known about their pattern of inheritance. Lack of atrial and ventricular septum closures counts for a significant percentage of all congenital heart defects. Individuals harboring such defects always suffer poor quality of life. The only way to help those patients is cardiac surgery. Several genes have been implicated in the process of controlling heart development. Recent studies on transgenic mice null for tolloid-like 1 (tll1) gene revealed factors possibly involved in signaling pathways that, when absent, might affect heart development and cause problems mimicking those observed in atrial septal defects (ASDs). Lack of activity of this metalloprotease caused incomplete closure of the septum in murine fetal hearts and led to death in mid-gestation due to severe circulation problems. tll1 gene is located on the long arm of chromosome 4 (4q32-q33). The gene comprises of 6654 nucleotides and it encodes a protein 1013 amino acids long. Its human homologue tll1 is poorly investigated and until now no particular disorders have been linked to mutations in this gene.