Gauge Theory Amplitudes In Twistor Space And Holomorphic Anomaly
  We show that, in analyzing differential equations obeyed by one-loop gauge theory amplitudes, one must take into account a certain holomorphic anomaly. When this is done, the results are consistent with the simplest twistor-space picture of the available one-loop amplitudes. 
Freddy Cachazo, Peter Svrcek, Edward Witten