An object to be solved by the present invention is to identify patients resistant to known HSP90 inhibitors, and to provide a novel therapeutic agent for treating the patients who have become resistant to known HSP90 inhibitors. As a means for solving the above problems, the present invention provides identification of the patients based on a protein, which is an HSP90 family protein having a mutation in the site corresponding to F138 of HSP90± class A consisting of the amino acid sequence of SEQ ID NO: 1, and use of a substance that inhibits the protein as an active ingredient of a therapeutic agent.
