Overkill Software has announced they're working with Valve on an "in-depth" collaboration pertaining to Payday: The Heist and Left 4 Dead.
A possible collaboration was hinted at within Payday: The Heist, and studio game director Ulf Andersson has confirmed exactly that:
"As perceptive gamers will have noticed, several hints have recently been dropped into Payday The Heist, which has led to various rumors. We are excited to be able to confirm that an in-depth collaboration between Overkill and Valve is currently in production.
"We are working on a very cool blend of PAYDAY and Left 4 Dead. I am sure it is so exciting that it will have some players check into the hospital before we are done."
Overkill were bought by Syndicate developer Starbreeze, which was followed by a confirmation of a Payday: The Heist sequel. The PC and PSN title itself, meanwhile, has sold 400,000 units since its October release.