Cooperativity of Protein Folding and the Random-Field Ising Model
  The relation between cooperativity of protein folding and the Random-Field Ising Model (RFIM) is established. Generalization of the Imry-Ma argument predicts cooperative folding transition for small heterogeneity of the interactions stabilizing the native structure. Monte Carlo simulation of a lattice model shows that starting from some finite heterogeneity folding transition is not cooperative and involves formation of domains. 
A. M. Gutin, V. I. Abkevich, and E. I. Shakhnovich (Harvard   University, Department of Chemistry, Cambridge, MA)