While A, T, C, and G represent a particular nucleotide at a position, there are also letters that represent ambiguity which are used when more than one kind of nucleotide could occur at that position.
虽然A，T，C和G代表某个位置的特定核苷酸，但也有代表模糊性的字母，当在该位置可能出现一种以上的核苷酸时使用这些字母。