Philadelphia's District Attorney has appointed a dogged prosecutor to lead a new unit that will double-check major cases under appeal or review. 

Mark Gilson is known as the "pitbull" or "bulldog" depending on whom you talk to in the DA's office. He's a second generation prosecutor who has taken on major, challenging cases and won. Now Philadelphia District Attorney Seth Williams says Gilson will be reviewing cases to make sure the prosecution did the right thing.

"His investigative techniques and knowledge of the law are above reproach," Williams said. "This new position is designed to balance the office's interest of investigating good faith claims of innocence with the obligation to defend valid convictions."

Marissa Bluestine of the Pennsylvania Innocence Project says the group has about 17 questionable cases under appeal statewide, about 10 of those in Philadelphia. She's pleased with the DA's decision saying there is a nationwide trend to make sure a conviction is correct.

"Thirty-three of the 87 exonerations were done with the cooperation or by the initiative of law enforcement," Bluestine said. "This is on the cutting edge of identifying those who have been convicted of crimes they did not commit. Let me be clear, with the Pennsylvania Innocence Project that is our goal."

Gilson says he doesn't believe his new job will be any different from prosecuting major cases since he has handled many retrials in the past with some re-investigation.

"I have a lot of experience with prosecuting cases which means I have a lot of experience looking at evidence. I have a lot of connections with law enforcement. I think the job I'm doing is an extension of the job I've been doing for the past 27 years. I don't think it's going to be any problem whatsoever."