UGI relocation inside Cas9 reduces Cas9 dependent off target effects in cytosine base editors.

Cytosine base editors (CBEs) achieve precise C-to-T conversions by addition of uracil DNA glycosylase inhibitor (UGI) with Cas9 nickase (nCas9) and cytidine deaminase, and the conventional fusion at the nCas9 carboxyl terminus effectively inhibits uracil excision repair to enhance editing efficiency. However, despite potent on-target activity, classical CBEs exhibit significant Cas9-dependent DNA off-target effects that necessitate optimization for future applications. Here we present a strategic UGI relocation through internal fusion within the nCas9 architecture. This spatial reorganization maintains comparable on-target editing efficiency while substantially reducing Cas9-dependent DNA off-target activity. Our findings establish an alternative engineering paradigm to develop high-fidelity CBEs, offering an improved platform for widespread genome editing applications.