There are more than 20 amino acids that make up the biological polymers called proteins, so there are 20 choices of each amino acid at a position on a protein.

而那些称为蛋白质的生物活性多聚体,是由二十种不同氨基酸所构成,因此每个蛋白质的,任意位点都有多达二十种氨基酸可供选择

耶鲁公开课 - 生物医学工程探索课程节选