Next, we replaced wild-type St1Cas9 of CTBEWT with the KQKL variant to expand the editing scope of the C-to-T base editor (Fig. ). The resulting CTBEevolve system was first subjected to ATc concentration optimization because ATc is the inducer for the expression of the engineered St1Cas9 variant. We screened six different concentrations of ATc ranging from 5 ng/mL to 100 ng/mL and no significant differences in the editing efficiencies were noticed (Fig. ). We selected 20 ng/mL as the inducer concentration for base editing in the subsequent experiments.