The present invention relates to polypeptides which comprise the ligand binding domain of Lck, crystalline forms of these polypeptides, and the use of these crystalline forms to determine the three dimensional structure of the catalytic domain of Lck. The invention also relates to the use of the three dimensional structure of the Lck catalytic domain both alone, or in complex with inhibitors, in methods of designing and/or identifying potential inhibitors of Lck activity, for example, compounds which inhibit the binding of a native substrate to the Lck catalytic domain. The invention also relates to the use of the three dimensional structure of the Lck catalytic domain both alone, or in complex with inhibitors, in methods of designing and/or identifying potential selective inhibitors of Lck activity, for example, compounds which inhibit the binding of a native substrate to the Lck catalytic domain selectively.