If the warhead is sufficiently selective to react with a desired residue, the selectivity of the guidance system for a particular protein and its strength of binding to the protein determine the rate of protein-inhibitor complex formation. The position of the warhead with respect to the target residue in the protein-inhibitor complex and the reactivity of the warhead determine the rate of inhibition. As noted earlier, irreversible inhibitors can be assayed using mass spectrometric methods. Phenotypic methods look for a desired biological activity directly, while mass spectrometric methods (activity-based protein profiling (ABPP) and isotopic labeling) allow scientists to identify residues that can be modified or are modified by specific inhibitors. Computational methods to model inhibitor binding and protein structure are useful in estimating the positioning of reactive groups in protein-inhibitor complexes. In many cases, however, the underlying protein structural data to determine binding modes and selectivities must be validated by experimental data. If the desired pharmacokinetic behavior for enzyme inhibition is known, kinetic data for warheads can be used to determine appropriate inhibitor if the rate of enzyme resynthesis is known.