A new method for programmable base editing in genomic DNA without double-stranded DNA cleavage has been developed. This approach enables the direct, irreversible conversion of one DNA base to another at a target locus, without requiring DNA backbone cleavage or a donor template. The method involves fusing CRISPR/Cas9 with a cytidine deaminase enzyme, which allows for the conversion of cytidine to uridine, resulting in C→T or G→A substitutions. This technique was tested in human and murine cell lines, where second- and third-generation base editors, which include a uracil glycosylase inhibitor (UGI) and a Cas9 nickase, were used to manipulate the DNA repair response to favor desired base-editing outcomes. These editors achieved permanent correction of 15-75% of total cellular DNA with minimal indel formation. Base editing expands the scope and efficiency of genome editing for point mutations. The method was also tested in mammalian cells for correcting disease-relevant mutations, such as in the apolipoprotein E gene and the p53 mutation, demonstrating high efficiency and precision. The development of base editing offers a more efficient and less error-prone alternative to traditional genome editing methods, with the potential to expand the range of genome modifications that can be cleanly installed.A new method for programmable base editing in genomic DNA without double-stranded DNA cleavage has been developed. This approach enables the direct, irreversible conversion of one DNA base to another at a target locus, without requiring DNA backbone cleavage or a donor template. The method involves fusing CRISPR/Cas9 with a cytidine deaminase enzyme, which allows for the conversion of cytidine to uridine, resulting in C→T or G→A substitutions. This technique was tested in human and murine cell lines, where second- and third-generation base editors, which include a uracil glycosylase inhibitor (UGI) and a Cas9 nickase, were used to manipulate the DNA repair response to favor desired base-editing outcomes. These editors achieved permanent correction of 15-75% of total cellular DNA with minimal indel formation. Base editing expands the scope and efficiency of genome editing for point mutations. The method was also tested in mammalian cells for correcting disease-relevant mutations, such as in the apolipoprotein E gene and the p53 mutation, demonstrating high efficiency and precision. The development of base editing offers a more efficient and less error-prone alternative to traditional genome editing methods, with the potential to expand the range of genome modifications that can be cleanly installed.