Starting adenine deaminases (used for tRNA processing), so

Starting in 2016, the above hybrid ‘Target-AID’ or ‘CRISPR Nickase System’ of cytosine deaminases was extensively used for developing a large
number of plasmid vectors10, which were described as base editors
(BEs). These BEs were successfully used for generating precise CG®TA mutations in mouse/ Xenopus/human
living cells and embryos (Figure 3). In case of mouse and Xenopus, the modified embryos were also used for transplantation in
pseudopregnant surrogate mothers for generating offsprings exhibiting altered
phenotypes (e.g., black to albino body colour). In plant systems also,
protoplasts were successfully used for creating mutations in specific genes,
and then regenerated into whole plants exhibiting desirable altered traits. The progress made in the development and use of BEs was also summarized
in five articles that appeared in a special issue of Nature
Biotechnology in May 2017 (see later). These five papers included papers on base editing in mouse embryos as well as in
crops including rice, wheat, maize and tomato. The work on C®T base editing was followed by two
important reports that were simultaneously published in October 2017. In one
report, DNA adenine deaminases, which did not occur in nature were developed in
the laboratory using naturally occurring RNA adenine deaminases (used for tRNA
processing), so that a series of adenine base editors (ABE) became available for
conversion of adenine into inosine in a DNA molecule. Thus it also became
possible to replace A:T base pair by G:C (A:T®I:T®G:C; Figure
3)11. In the second report, a technology for base editing of RNA
transcripts was published12. In this manner, CRISPR/Cas was modified
and used for editing DNA/RNA molecules in the form of BEs and ABEs. This brought
mani-fold efficiency
and precision to gene editing technology.