The invention is directed to methods, reagents, and kits for the detection of MAPK/ERK pathway mutations in a patient diagnosed for cancer. In one embodiment, the invention comprises a sensitive and selective method to identify mutations to the BRAF, KRAS, and NRAS genes in a single reaction. In another aspect of this embodiment, the invention comprises primers and probes for the detection of the BRAF, KRAS, and NRAS mutations using a single nucleotide primer extension assay. In another embodiment the invention is used to identify and select patients amenable for treatment with an ERK inhibitor.