More accurately, skin cells. It was developed for treating severe burn victims.http://www.cracked.com/article_19476_5-sci-fi-medical-procedures-well-have-in-our-lifetime.html
They have an example of someone who had bad second-degree burns across a large part of his body, who was actually being interviewed as part of the video clip where they explained the gun. He didn't even look like he'd been burned. Apparently, it's been used on a dozen people already, with no ill effects, since they use the person's own skin stem cells to do the trick.