The present inventors used antibody engineering techniques to prepare functional antibodies that correspond to individual mutations in causative genes of diseases, and discovered that such antibodies enable the treatment of the diseases. Specifically, the inventors succeeded in preparing ligands, particularly minibodies, which have agonistic activity to receptors that have almost completely lost responsiveness to their natural ligands because of gene mutations (for example, a thrombopoietin (TPO) receptor whose reactivity to TPO has been markedly impaired), and which can transduce signals by interacting with these mutant receptors at levels comparable to normal.