One-Loop Gauge Theory Amplitudes with an Arbitrary Number of External   Legs
  We review recent progress in calculations of one-loop QCD amplitudes. By imposing the consistency requirements of unitarity and correct behavior as the momenta of two legs become collinear, we construct ansatze for one-loop amplitudes with an arbitrary number of external legs. For supersymmetric amplitudes, which can be thought of as components of QCD amplitudes, the cuts uniquely specify the amplitude. 
Z. Bern, L. Dixon, D. Dunbar, and D.A. Kosower