Contrary to what viewers of “CSI” might believe, not every criminal case can be resolved by DNA evidence. Even so, DNA testing has exonerated more than 200 convicted defendants. This week, the Supreme Court was asked to rule that access to DNA information is a constitutional right. The court must say yes.

Sixteen years ago, William Osborne was convicted in an Alaska court of raping a prostitute. In oral arguments Monday, Alaska defended its decision to deny Osborne an advanced DNA test, which could establish his innocence or his guilt. Alaska is one of six states that don’t provide convicted defendants access to DNA evidence, although Kenneth Rosenstein, Alaska’s lawyer, said Osborne might be able to obtain such evidence if he insisted in advance that he is innocent -- something he hasn’t done.

Osborne is, to put it mildly, an unsympathetic representative of prisoners seeking access to DNA information. He filed his claim under a civil rights law that seems only tenuously linked to his situation. He declined a sophisticated DNA test during his trial -- leading Justice Samuel A. Alito Jr. to suggest that he was “gaming the system.” He also confessed to the crime in parole hearings. Still, he is entitled to the evidence he seeks.

Alaska’s argument that there is no constitutional right to DNA evidence was supported by the Obama administration. Deputy Solicitor General Neal Katyal noted that federal law requires that a defendant seeking DNA evidence swear that he is innocent. Besides, Katyal said, the Supreme Court never has said explicitly that a defendant convicted after a fair trial has a constitutional right to challenge that conviction on the grounds that he is innocent.

In this or some other case, the court should rule that convicted defendants should be allowed to show that new evidence proves their innocence. But even if it doesn’t go that far, it should recognize that DNA evidence is uniquely capable of establishing guilt or innocence, a fact reflected in the abundance of state laws on the subject, including one in California.

States would still be free to establish rules to prevent gaming the system, such as a requirement that a convicted defendant swear under oath that the evidence will exonerate him. That would minimize the danger of what Alaska’s lawyer called “fishing for evidence.” The alternative is to say that a state may withhold evidence that could free a wrongly convicted person. That’s an inconceivable interpretation of the Constitution’s guarantee of due process of law.