 Overproduction of red blood cells increases a chance of adverse cardiovascular event, such as thrombosis and stroke.Rarely, seemingly beneficial mutations in the EpoR may arise, where increased red blood cell number allows for improved oxygen delivery in athletic endurance events with no apparent adverse effects upon the athlete's health (as for example in the Finnish athlete Eero Mäntyranta).Erythropoietin was reported to maintain endothelial cells and to promote tumor angiogenesis, hence the dysregulation of EpoR may affect the growth of certain tumors.