For protein expression and screening, all transfer and aliquotting steps were performed using Hamilton liquid-handling robots. Chemically competent E. coli BL21(DE3) cells were transformed with the ligated libraries. Freshly transformed clones were used to inoculate 150 μL of LB medium supplemented with 50 μg mL -1 kanamycin in Corning® Costar® 96-well microtitre round bottom plates. For reference, each plate contained 6 freshly transformed clones of the parent template and 2 clones containing an empty pET29b(+) vector. Plates were incubated overnight at 30 °C, 80 % humidity in a shaking incubator at 850 r.p.m. 20 µL of overnight culture was used to inoculate 480 μL 2xYT medium supplemented with 50 μg mL -1 kanamycin. The cultures were incubated at 30 °C, 80 % humidity with shaking at 850 r.p.m. until an OD600 of about 0.5 was reached, and IPTG was added to a final concentration of 0.1 mM. Induced plates were incubated for 20 h at 30 °C, 80 % humidity with shaking at 850 r.p.m. Cells were harvested by centrifugation at 2,900 x g for min. The supernatant was discarded, the pelleted cells were resuspended in 400 μL lysis buffer (PBS pH 7.4 buffer supplemented with 1.0 mg mL -1 lysozyme, 0.5 mg mL -1 polymixin B and 10 μg mL -1 DNase I) and incubated for 2 h at 30 °C, 80 % humidity with shaking at 850 r.p.m. Cell debris was removed by centrifugation at 2,900 x g for 10 min. 10 µL clarified lysate was transferred to 96-well microtitre plates containing 170 µL 1% NH2OH. Reactions were initiated with the addition of 20 µL cytidine (50 mM final concentration) in PBS pH 7.4. The reaction was monitored spectrophotometrically at 340 nm, over 30 minutes using a CLARIOstar plate reader (BMG Labtech). Reaction rates of individual variants were normalized to the average of the 6 parent clones. Following each round, the most active variants were rescreened as purified proteins using the HPLC assay. Proteins were produced and purified as described above, however starter cultures were inoculated from glycerol stocks prepared from the original overnight cultures.