We propose a simple algorithm to design a sequence which fits a given
protein structure with a given energy. The algorithm is a modification
of the Metropolis Monte Carlo scheme in sequence space with an evolut
ionary temperature which sets the energy scale. There is a one to one
correspondence between this optimization scheme and the Ising model of
ferromagnetism. This analogy implies that the design algorithm does n
ot encounter multiple-minima problems and is very fast. The algorithm
is tested by 'predicting' the primary structures of four proteins. In
each case the calculated primary structures had statistically signific
ant homology with the natural structures.