 The hydrogen bond between Asp2 and the N4 of either a cytosine or adenine base paired to the guanine or thymine, respectively defines these two nucleotides at the 3' position, defining a sequence that overlaps into the subsite of any zinc finger that may be attached N-terminally.Target-site overlap limits the modularity of those zinc fingers which exhibit it, by restricting the number of situations to which they can be applied.If some of the zinc fingers are restricted in this way, then a larger repertoire is required to address the situations in which those zinc fingers cannot be used.