RAF1 and cancer: While K27 inhibits the RAF/MEK/ERK pathway in the three RAS mutant cell lines and the RASWT cancer cell line tested (consistent with its binding to the switch regions), K13 and K19 only decrease significantly the phosphorylation of MEK and ERK kinases in the KRAS mutant HCT116 cell line, but not in either the NRAS or the HRAS mutant cell lines or the RASWT cancer cell line (Fig. 5a, b).