At the age of thirty-three, he became one of the youngest doctors in the United States to hold that position.

VOA: special.2009.09.04

Because it takes three units where there's only four choices at each position to have at least 20 unique combinations.

因为每个位点上仅有四种可能性,只有采用三个碱基一组,才能取得至少二十种不同的组合方式

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