Funny enough, I know a little bit about this one. We have them, but they are all come-from-behind missiles, so the launch site has to be as close to Russia (or wherever) as possible, which angers the Russians.
The head-on hit-a-bullet-with-another-bullet-is missile still impossible, although many would like us to believe they are already here. Nuh uh.
Plus those are still useless against nukes delivered inside ton of cocaine.