Cryonics is the low-temperature preservation of humans and animals who cannot be sustained by contemporary medicine, with the hope that healing and resuscitation may be possible in the future. Cryopreservation of people or large animals is not reversible with current technology. The stated rationale for cryonics is that people who are considered dead by current legal or medical definitions may not necessarily be dead according to the more stringent information-theoretic definition of death. It is proposed that cryopreserved people might someday be recovered by using highly advanced future technology. The future repair technologies assumed by cryonics are still hypothetical and not widely known or recognized. Responding to skepticism from scientists such as Steve Jones, an open letter supporting cryonics was written and signed by currently 61 scientists. As of 2012, only around 250 people have undergone the procedure since it was first proposed in 1962. In the United States, cryonics can only be legally performed on humans after they have been pronounced legally dead as otherwise it would be considered murder or assisted suicide. Cryonics procedures ideally begin within minutes of cardiac arrest, and use cryoprotectants to prevent ice formation during cryopreservation. However, the idea of cryonics also includes preservation of people after longer post-mortem delays because of the possibility that brain structures encoding memory and personality may still persist or be inferable. Whether sufficient brain information still exists for cryonics to work under some preservation conditions may be intrinsically unprovable by present knowledge. Therefore, most proponents of cryonics see it as an intervention with prospects for success that vary widely depending on circumstances.