Chemical conversion of individual bases — no double-strand breaks, no template
Editing Window
Target sequence (positions 1-20):
CBE — Cytosine Base Editor
CBE (e.g., BE4max): nCas9 (D10A nickase) fused to a cytidine deaminase (rAPOBEC1 or evoAPOBEC). A UGI (uracil glycosylase inhibitor) domain prevents U excision.
Mechanism: nCas9 unwinds DNA, exposing single-stranded target. Deaminase converts C→U in the editing window (positions ~4-8 from PAM-distal end). nCas9 nicks non-edited strand, biasing repair to install C•G → T•A at target.
No DSB, no donor template required.
Precision vs CRISPR
CRISPR-Cas9: creates DSB → NHEJ (indels) or HDR (needs template)
Base editing: creates point mutation only, no indels (low), no template. Ideal for correcting pathogenic point mutations (e.g., sickle cell: β-globin Glu6Val).
Limitation: can only convert C→T (CBE) or A→G (ABE) — not all 12 transversion types yet.