third, downstream effectors of mitogenic signaling are often mutated in cancer cells. an important mitogenic signaling pathway in humans is the ras-raf-mapk pathway. mitogenic signaling normally activates ras, a gtpase, that then activates the rest of the mapk pathway, ultimately expressing proteins that stimulate cell cycle progression. it is likely that most, if not all, cancers have some mutation in the ras-raf-mapk pathway, most commonly in ras. these mutations allow the pathway to be constitutively activated, regardless of the presence of mitogens.