After 4. a4, Black has three replies. The most popular line is the "Sosonko Accepted" variation, where Black plays 4... bxc4. This often leads to sharp and sacrificial lines for both sides. Such lines include the "poisoned knight" variation where after 4. a4 bxc4 5. Nc3 e6 6. e4 exd5 and 7. e5, Black sacrifices a knight for a large central pawn majority and excellent advantage with good attacking chances; and the "River Styx Attack", which continues 5. Nc3 Ba6, 6. e4 d6, 7. f4 g6, 8. e5. This leads to a sharp and complicated pawn sacrifice by White, where White often delays or even prevents Black from castling, and has a solid grip over the kingside with the e6-pawn and Nf7 outpost combination. It is named after the Greek mythological river that interconnects the Earth and the Underworld.