This invention provides methods and compositions for preventing, treating or ameliorating one or more symptoms of a malignant tumor associated with KRAS mutation in a mammal in need thereof, by identifying a tumor cell in the mammal, the tumor cell comprising at least one of: (i) a mutation of the KRAS gene, and (ii) an aberrant expression level of KRAS protein; and administering to the mammal a therapeutically effective amount of a composition comprising one or more RNAi molecules that are active in reducing expression of GST-π.