1. d2d4  1... g8f6  2. c2c4  2... e7e6  3. b1c3  3... d7d5  4. c4d5  4... e6d5  5. c1g5 World championship matches give a popularity boost to the lines that were explored there. 5... c7c6  6. e2e3  6... h7h6 