KRAS and metastatic melanoma: RAS or RAF mutations occur in 47% of metastatic melanoma cases,46 and KRAS mutations are found in ~25%–27% of human lung adenocarcinomas.47 RAS, RAF, and MEK mutations, however, are rarely observed—at rates of ~1%–6%—in breast cancer.48 Thus, it is plausible that in breast cancers, ERK activation may proceed via persistent upstream signaling rather than constitutively active RAS, RAF, or MEK mutants.