I assume you are talking about a PnP version of the video game series. As far as I have been able to find there really isn't a game for it, but any post apocalyptic capable game will work. I would think the D20 modern apocalypse game would be a good start. I know GURPS is capable of handling it, but I've always had trouble dealing with GURPS games. I heard some talk about Leviathan being a good post apoc. game and that may work as well.