The protein GDF11 is found in humans as well as mice, and is now being considered for possible human testing due to its surprising and apparently regenerative properties.
A previous study had focused on examining the hearts of mice the equivalent of 70 human years old. The mice were regularly exposed to the blood of younger mice whose blood carried a higher concentration of GDF11. Ordinarily the hearts of older mice are enlarged and weakened, however results from the previous study displayed that, thanks to the GDF11 protein present in the blood of the younger mice, the hearts of the elderly mice reduced in size, making them appear younger and healthier. The changes were not purely aesthetic however, with the mice displaying an increased ability to exercise for prolonged periods of time.