first, cancer cells can produce their own mitogens, a term called autocrine stimulation. this can result in a deadly positive feedback loop - tumor cells produce their own mitogens, which stimulate more tumor cells to replicate, which can then produce even more mitogens. for example, consider one of the earliest oncogenes to be identified, p28sis from the simian sarcoma virus, which causes tumorigenesis in the host animal. scientists found that p28sis has a nearly identical amino acid sequence as human platelet-derived growth factor (pdgf). thus, tumors formed by the simian sarcoma virus are no longer dependent on the fluctuations of pdgf that control cell growth; instead, they can produce their own mitogens in the form of p28sis. with enough p28sis activity, the cells can proliferate without restriction, resulting in cancer.