Cardiovascular system consisting of the heart and blood vessels, which is established in early embryogenesis, is a very important biological system. The impairment of cardiovascular system is related to the main causes of death (cancer, heart disease and cerebrovascular disease) in Japanese. Therefore, the elucidation of molecular mechanism of cardiovascular formation in vertebrates is indispensable for both the pathologic understanding of human cardiovascular diseases and the development of therapeutic drugs in the diseases. Our research group is performing the functional analysis of zebrafish mutants defective in the cardiovascular system by using the fluorescent proteins visualizing the cardiovascular formation. Further, we will clarify whether the molecular function of the cardiovascular key regulator identified from the zebrafish mutant is conserved in mammals including humans.