Illustration of excess red blood cells (erythrocytes, red discs) and platelets (white) caused by polycythaemia vera, a rare blood cancer involving a mutation in the JAK2 gene. Also seen are healthy white blood cells (translucent/pink). The excess erythrocytes thicken blood, impairing circulation. They also increase the risk of complications such as blood clots, which can lead to a heart attack or stroke. Without treatment, polycythaemia vera can be life-threatening. Treatment is typically aimed at reducing erythrocyte count, but varies from case to case.