An assay of identifying the responsiveness of a HER2-positive cancer in a patient to a HER2 targeted drug, the method comprising the step of comparing a level of NeuromedinU in a biological sample obtained from the patient with a reference level of NeuromedinU, wherein detection of a level of NeuromedinU that is increased compared to the reference level of NeuromedinU indicates that the HER2-positive cancer has reduced responsiveness to a HER2 targeted drug.