This research looks at two conditions, Essential Thrombocythemia (ET) and Polycythemia Vera (PV). ET causes people to produce too many blood cells called platelets and PV causes too many platelets and red blood cells to be made. Platelets are particles which circulate in the blood stream and normally prevent bleeding and bruising. Having too many platelets in the blood increases the risk of developing blood clots, which can result in life threatening events like heart attacks and strokes. When the number of red blood cells is increased in PV this will slow the speed of blood flow in the body and increase the risk of developing blood clots.