Myosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes. Myosin heavy chains are the myosin type responsible for muscle contraction in muscle cells. Defects in MYH11 are the cause of aortic aneurysm familial thoracic type 4 (AAT4); also known as familial thoracic aortic aneurysm and dissection (TAAD). Aneurysms and dissections of the aorta usually result from degenerative changes in the aortic wall.