Starting this year, every convicted felon in Pennsylvania has been required to provide a tissue sample for DNA testing.

The goal is to see whether the prisoner's genetic profile matches the DNA from any of nearly 120,000 unsolved cases in a national database.

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This procedure, which started on a smaller scale in 1995, so far has matched 321 Pennsylvania prisoners to previously unsolved cases, according to Christine Tomsey, Pennsylvania State Police DNA Crime Lab director.

But the law has a blind spot -- it does not require that the prisoner's DNA be compared to evidence from his original case to make sure he was properly convicted.

There are practical reasons why that might not be a good idea, experts say.

For one thing, it would place an extra burden on an already overloaded system, Ms. Tomsey said. When the law was rewritten last year to require every felon to provide a DNA sample, she said, the state lab quickly went from having no backlog to falling behind on 49,500 specimens.

Ms. Tomsey said she hopes to get a federal grant to shift backlogged cases to an outside lab, but in the meantime, she expects another 50,000 samples to roll into her lab over the next year.

William Thompson, a criminology professor at the University of California at Irvine, said another reason not to test felons' DNA automatically against evidence in their original crimes is that many know they are guilty and wouldn't ask for such a test if it were up to them.

Dr. Thompson said a better policy is to make it as easy as possible for prisoners who believe they were wrongly convicted to get DNA testing done.

Pennsylvania has had a post-conviction DNA testing law in effect since 2002, but many genetic testing advocates say it is not working well.

Like most such laws around the United States, it contains criteria to help judges decide which convicts are entitled to post-conviction testing.

Some believe those criteria are unfair.

For instance, the law does not allow prisoners on parole or those who pleaded guilty to their crimes to seek DNA testing.

When a prisoner is on parole, said David Rudovsky, a law professor at the University of Pennsylvania who has handled DNA litigation, "you can be sent back to prison; your liberty is restricted. Why should we keep someone on parole if they are innocent?"

Denying DNA testing to prisoners who pleaded guilty also is unfair, he said.

"People plead guilty for all kinds of reasons -- a lot of times, if you get a good enough deal [on prison time], you'll plead guilty, even if you're innocent."

Pennsylvania's law also limits DNA requests to prisoners who were convicted before 1995. The exception is cases in which evidence was discovered after a trial concluded.

The rationale is that modern DNA technology has been around since 1995, so anyone convicted in the last 10 years would have had access to DNA testing if it had been relevant in their cases.

But John Rago, a law professor at Duquesne University and director of the Cyril H. Wecht Institute of Forensic Science and Law, said this ignores the possibility that DNA tests presented at trial could have been flawed or fraudulent.

The law also says that the prisoner's identity and involvement in the crime must have been at issue during his trial.

Mr. Rago said some judges recently have denied DNA testing because they thought witness testimony clearly showed guilt.

The problem with that, said Mr. Rago and other experts, is that studies have shown that witness recollections in criminal cases can be highly unreliable. In fact, in a number of high-profile cases around the nation in which DNA testing established innocence, witnesses have continued to insist that the exonerated man committed the crime.

An investigation by the Pittsburgh Post-Gazette and the Innocence Institute of Point Park University published this week also showed how prosecutors often argue against DNA testing unless it would conclusively prove an inmate's "actual innocence."

Sen. Stewart Greenleaf, R-Montgomery, the author of the post-conviction DNA law, said he plans to hold hearings early next year to determine if amendments are needed. His goal, he said, is to make sure that every eligible prisoner who wants DNA testing can get it.

So far, Mr. Rago said, only two convicts have been exonerated through DNA testing under the 2002 law. DNA tests freed six other Pennsylvania inmates who filed appeals before the 2002 law went into effect.

Rep. Kathy Manderino, D-Philadelphia, a member of the House Judiciary Committee, said she looks at DNA testing as a sharp tool that can cut both ways.

She said one concern she has about expanding post-conviction DNA testing is the cost. Mr. Rago shares that concern.

"If every DNA test costs $500 to $700 on average, it would be too big a burden to test everyone" against evidence from their original trials, he said. On the other hand, he noted, the cost of testing needs to be weighed against the current expense of more than $28,000 a year to house an inmate in state prison.

Mr. Rago favors granting post-conviction DNA tests any time there are allegations of certain types of problems in the trial.

In most cases where DNA has exonerated a prisoner, he said, research has shown there were often problems with unreliable witness testimony, coerced confessions, misconduct by police or prosecutors, poor scientific methods used on physical evidence, or testimony by an informant who received a deal from the prosecution.

Dr. Thompson said national surveys have shown that in states with lenient standards for post-conviction testing, there has not been a flood of prisoner requests, as some prosecutors had feared. About half of such DNA tests have exonerated the prisoners requesting them, he added.

"So suppose each test costs as much as $5,000," Dr. Thompson said, "and half of those got someone out of jail who didn't deserve to be there.

"Is it worth $10,000 a pop to get someone who's innocent out of prison? I think it might be worth 10 times that.

"False convictions," he said, "have a cost to society that is clearly much higher than the cost of testing to find them."