Mark Ruffalo is all set to play twin brothers in HBO's upcoming limited series 'I Know This Much Is True'. 'The Avengers' star will also serve as an executive producer on the series, which is based on Wally Lamb's New York Times bestseller of the same name, reports Deadline. The six-episode series will follow the parallel lives of identical twin brothers - Dominick and Thomas Birdsey - in an epic story of betrayal, sacrifice, and forgiveness, set against the backdrop of 20th century America. The project will be written, directed and executive produced by Derek Cianfrance (The Place Beyond the Pines). Meanwhile, Ruffalo's film credits include 'You Can Count on Me,' 'Eternal Sunshine of the Spotless Mind,' 'Collateral,' 'Spotlight,' 'The Kids Are All Right,' Infinitely Polar Bear,' 'Now You See Me,' and 'Foxcatcher.'