Provided are: a method of predicting sensitivity to a molecularly targeted drug a method of selecting a patient who is determined to have high responsiveness to administration of a drug and a reagent to be used in these methods. Specifically, provided are: a method of predicting responsiveness to cancer disease treatment with a compound that inhibits a mitogen-activated protein kinase (hereinafter abbreviated as MAPK) signaling pathway, the method including using a biological sample derived from a cancer patient, measuring whether or not β-catenin contained in the biological sample has at least one kind of mutation selected from the group consisting of (i) an active mutation and (ii) a substitution mutation of an asparagine residue to a serine residue at position 287 a method of selecting a patient to be subjected to cancer disease treatment with the compound a method of treating a cancer disease, the method including administering the compound to the patient selected by the above-mentioned method and a reagent to be used in these methods.
