Changes to influenza viruses can happen as a result of quick mutations or as the end result of a slower drift in genetic material.
发生在流感病毒上的变化，可能源于某种快速变异，也可能是遗传物质缓慢漂移的最终结果。
A new report says human embryonic stem cells kept for long periods of time develop changes in their genetic material.
一份新报告指出，长期保存的人类胎儿干细胞，其基因物质会逐渐产生变化。
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