Equilibrium correlations in a model for multidimensional epistasis
  We investigate a statistical model for multidimensional epistasis. The genotype is devided into subsequences, and within each subsequence mutations which occur in a prescribed order are beneficial. The bit-string model used to represent the genotype, may be cast in the form of a ferromagnetic Ising model with a staggered field. We obtain the actual correlations between mutations at different sites, within an equilibrium population at a given "tolerance," which we define to be the temperature of the statistical ensemble. 
G. Soyler and A. Erzan