Exoneration occurs when a person who has been convicte of a crime is later proved to have been innocent of that crime. Attempts to exonerate convicts are particularly controversial in death penalty cases, especially where new evidence is put forth after the execution has taken place. The first convict from a United States' prison to be released on account of DNA testing was David Vasquez, in 1989. Recently, DNA evidence has been used to exonerate a number of persons either on death row or serving lengthy prison sentences. As of October, 2003, the number of states authorizing convicts to request DNA testing on their behalf, since 1999, has increased from two to thirty. Access to DNA testing varies greatly by degree; post-conviction tests can be difficult to acquire. Organizations like the Innocence Project are particularly concerned with the exoneration of those who have been convicted based on weak evidence. As of October 2003, prosecutors of criminal cases must approve the defendant's request for DNA testing in certain cases. In other contexts, to exonerate can mean simply to free somebody from blame or guilt: to declare officially that somebody is not to blame or is not guilty of wrongdoing.