Thrombosis comes about by interactions between genetic and environmental or acquired factors, or both, resulting in a blood clot at a specific time and location. Because the disease occurs in episodes, the interaction of the genetic and nongenetic risk factors is important. Cystinosis is a recessive disorder caused by deposits of cystine within the lysosomes of cells-that is, sac-like cell parts that contain various enzymes. Involvement of the kidneys remains the primary characteristic, eventually leading to renal failure. Of all of the risk factors that make it easier for blood clotting, a high level of a substance called homocysteine is of particular interest. Too much homocysteine in blood plasma is a common finding in patients with kidney failure, and it has been recently identified as an independent risk factor for diseases of the blood vessels.