Events in embryonic development that are relevant to cancer cells. Two possible reasons: (a) many changes to normal cells are required for cancer development; or (b) the probability of a single event increases over time. Bishop and varmus proposed that the ancestral form of rsv picked up a copy of the src gene from an infected cell by mistake: this copy was no longer regulated properly and now acted as an. 1979, weinberg and colleagues: human cells treated and transformed (turned into cancerous cells) by a, they proposed that the mutation chemical carcinogen carried an oncogene called ras . Since then, more than 200 human oncogenes have been discovered: note: proto-oncogene, normal, non mutant form that does not contribute to cancer. We are born with the normal allele (form) of each of these genes (almost always). One of the steps in cancer development is the change of a proto-oncogene into an.