From @ImaCake's comment¹ above, I learned that: > Cas9 cannot edit the DNA by itself, but you can add a second enzyme to it that is able to change a single bas

From @ImaCake's comment¹ above, I learned that: > Cas9 cannot edit the DNA by itself, but you can add a second enzyme to it that is able to change a single base of DNA from one letter to another. In theory this means you could target a single nucleotide somewhere in the ~3 billion letters of the genome. ..A single letter flipped from A to G or C to T. That seems like an answer to your question. The "resolution", or a single "unit", in a genome sequence is a nucleotide. > But it will still have unintended edits! As for the accuracy of CRISPR, two kinds of errors are mentioned. "Off-target" mutations that occur elsewhere in the sequence: > It is pretty trivial to test for these in whole genome sequencing, and it turns out you will get plenty of them. And mutations that occur in the same targeted location: > ..[The] base editor will actually target a region of about 5-6 bases around the intended target and will happily flip those bases too. ¹ https://news.ycombinator.com/item?id=24459427 https://news.ycombinator.com/item?id=24459427 --- On further reading, a single "unit" for double-stranded D/RNA is the base pair: > bp = base pair(s) — one bp corresponds to approximately 3.4 Å (340 pm) of length along the strand, and to roughly 618 or 643 daltons for DNA and RNA respectively. (A picometer, "pm", is 1×10^−12 m, or one trillionth of a meter.) @ImaCake does mention that CRIPSR is able to "target a single nucleotide", a single base of DNA. For single-stranded D/RNA, the nucleotide is the unit, abbreviated nt (or knt, Mnt, Gnt). https://en.wikipedia.org/wiki/Base_pair#Length_measurements https://en.wikipedia.org/wiki/Base_pair#Length_measurements