base editing
Base editing changes a single DNA letter into another without ever cutting the double helix apart. If older CRISPR is like cutting out a word and hoping the repair fixes it, base editing is like using a pencil to change one letter cleanly — turning a C into a T, for example — leaving the rest of the text untouched.
A base editor fuses a crippled Cas9 (one that binds DNA but cuts at most one strand) to an enzyme that chemically modifies a base. Guided to the target, it converts one base directly into another: cytosine base editors change C–G pairs to T–A, and adenine base editors change A–T to G–C. Because there is no full double-strand break, the messy repair that scrambles sequences is largely avoided.
This makes base editing especially good at fixing or installing point mutations — the single-letter changes behind many genetic diseases. Its limits are real: it can only make certain base-to-base swaps, it works within a small window of the target, and it can sometimes edit nearby unintended bases or RNA, so design and checking still matter.
Standard base editors perform transitions (such as C-to-T and A-to-G); they cannot make every possible base swap. Prime editing is more flexible but works differently.