epas1 is useful in high altitudes as a short term adaptive response. however, epas1 can also cause excessive production of red blood cells leading to chronic mountain sickness that can lead to death and inhibited reproductive abilities. some mutations that increase its expression are associated with increased hypertension and stroke at low altitude, with symptoms similar to mountain sickness. populations living permanently at high altitudes experience selection on epas1 for mutations which reduce the negative fitness consequences of excessive red blood cell production.