由于病毒可能会意外地启动细胞本身含有的致癌基因，因此这种方法生成的干细胞用在患者身上就不够安全。
Because the virus could accidentally switch on cancer genes, the cells would not be safe enough to use in patients.
研究人员在70年代早期取得了很大进展，他们发现导致癌症的癌基因在正常细胞里是不活跃的。
The researchers made great progress in the early 1970s, when they discovered that oncogenes, which are cancer-causing genes, are inactive in normal cells.
致癌基因是一种可将健康细胞变成癌细胞的基因。
Oncogenes are genes that can cause healthy cells to transform into tumor cells.
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