The second main move.
1. c2c4  1... g8f6  2. b1c3  2... e7e5  3. g1f3  3... b8c6  4. g2g3  4... c6d4 A relatively rare line, which Caruana also used some time ago. It seems as it is coming back into fashion these days at the top level. 5. f1g2  5... d4f3  6. g2f3 