A modern, interesting, and combative move to face the Najdorf Defense. The idea is to advance the pawn to g4, and White doesn't mind losing castling rights on the kingside, as the king goes the other way.
1. e2e4  1... c7c5  2. g1f3  2... d7d6  3. d2d4  3... c5d4  4. f3d4  4... g8f6  5. b1c3  5... a7a6 