The object of the invention is to provide a method for quantitatively assessing the degree of progression of a malignant neoplasm in a patient who has been medicated. Provided is a method for assessing the progression of the clinical state of a malignant neoplasm in a subject who has been administered with a medicine for treating the malignant neoplasm, the method being characterized by comprising: (1) a step of determining the ratio of DNA molecules having an activation mutation that serves as an activation marker for the medicine to DNA molecules having a normal marker gene in DNA in the blood from the subject; (2) a step of determining the ratio of DNA molecules having a resistance mutation that serves as a resistance marker for the medicine to DNA molecules having a normal marker gene in the DNA in the blood from the subject; and (3) a step of comparing a value obtained in the step (2) with a value obtained in the step (1) to thereby assess whether or not the malignant neoplasm in the subject has acquired resistance to the treatment with the medicine.
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