Phi value analysis, formula_1 analysis, or formula_2-value analysis is an experimental protein engineering technique for studying the structure of the folding transition state of small protein domains that fold in a two-state manner. The structure of the folding transition state is hard to find using methods like protein NMR or X-ray crystallography because folding transitions states are mobile and partly unstructured by definition. In formula_3-value analysis, the folding kinetics and conformational folding stability of the wild-type protein are compared with those of point mutants to find "phi values". These measure the mutant residue's energetic contribution to the folding transition state, which reveals the degree of native structure around the mutated residue in the transition state, by accounting for the relative free energies of the unfolded state, the folded state, and the transition state for the wild-type and mutant proteins.