Heart valve disease, which includes both congenital and acquired forms, is an important health problem in different regions of the world. It appears crucial to elucidate the etiology of this disease. Development and remodeling of the cardiac valves are complex processes that lead to mature valve leaflets composed of highly organized extracellular matrix consisting of three distinct layers made of collagens, proteoglycan and elastin. However, the factors that regulate matrix components required for the structure and biomechanical properties of the valves are unknown. Our lab shows that the zinc finger transcription factor Krox20 is expressed in the embryonic and postnatal semilunar and atrioventricular valves, and that ablation of Krox20 causes significant hyperplasia of the aortic and pulmonary valves. Targeted deletion of Krox20 in adult valves results in aortic regurgitation. These defective aortic valves have features of human myxomatous valve disease, including excess of proteoglycan deposition and reduction of collagen fibers, particularly collagens type I and III. Using functional promoter analyses and in vivo experiments, we conclude that Krox20-mediated activation of fibrillar Col1a1 and Col3a1 genes is crucial for normal function of the aortic valve. These results may help to understand the mechanism that lead to valve disease.