Abstract: The present invention relates to a method of selecting (a) cell(s), (a) tissue(s) or (a) cell culture(s) with susceptibility to an HSP90 inhibitor. Also a method for determining the responsiveness of a mammalian tumor cell or cancer cell to treatment with an HSP90 inhibitor is described herein. In particular, the present invention provides for an in vitro method for the identification of a responder for or a patient sensitive to an HSP90 inhibitor and uses of an oligo- or polynucleotide capable of detecting (an) activating mutation(s) in the KRAS gene are provided. The present invention also relates to a method of monitoring the efficacy of a treatment of a cancer characterized by the presence of at least one activating mutation in the KRAS gene, and. optionally, in the EGFR gene and/or the BRAF gene. In addition, a method of predicting the efficacy of a cancer treatment is described, in particular in a cancer that is characterized by the presence of at least one activating mutation in the KRAS gene. and.