For monoclonal screening after CRISPR editing, is limiting dilution or flow cytometry sorting more reliable? How many cells are needed for T7E1?

293T cells adhere well and have a high monoclonal formation rate, so limiting dilution is entirely feasible without a flow cytometer. My approach is to first collect pooled cells after transfection and extract gDNA for validation; after confirming the presence of editing signals, seed them into 96-well plates at a dilution of 0.3-0.5 cells per well, and the next day mark single-cell wells under the microscope well by well. The HDR efficiency for point mutations fluctuates greatly; homology arm length, whether the template is ssODN or dsDNA, and the transfection method all affect it. Empirically, it ranges from 1% to 10%, so do not set expectations too high. I usually prioritize ssODN as the template, with homology arms of 60-90 nt, and design a synonymous mutation restriction enzyme site near the mutation site to facilitate rapid subsequent preliminary screening.

For T7E1 enzyme, I usually use NEB's T7 Endonuclease I, aliquot it and store at -20°C, avoid repeated freeze-thaw cycles, and the stability is sufficient for routine screening. Takara and Thermo also have products with consistent quality; domestic enzymes with the same function are cheaper and arrive faster, but their cleavage efficiency should first be validated using positive PCR products. Before ordering, you can ask the distributor to send a small package; the lead time is generally 3-5 days, and the price is not expensive. For each digestion, always set up a wild-type PCR product control to determine nonspecific cleavage. Add the buffer and enzyme together according to the ratio in the instructions; do not change the conditions yourself, otherwise the bands will be difficult to interpret.

T7E1 cannot directly digest gDNA; you first need to PCR-amplify the fragment containing the editing site from the genome. I typically use 50-200 ng gDNA as template, then take 200-500 ng purified PCR product for annealing/reannealing and T7E1 digestion. If gDNA quality is poor or the fragment has high GC content, first optimize the PCR annealing temperature and extension conditions. If a point mutation introduces or disrupts a restriction enzyme site, it is recommended to prioritize RFLP or direct Sanger sequencing, because T7E1 is insensitive to small HDR edits and certain mismatches, making false negatives likely. T7E1 can be used as an initial screen, and positive candidates should then be confirmed by sequencing.

The biggest pitfall of limiting dilution is not the dilution itself, but multi-cell wells caused by errors in dissociation, pipetting, and counting. It is recommended to digest with 0.25% trypsin into a single-cell suspension, pass through a 40 μm strainer, and after counting, dilute to approximately 0.3 cells per 100 μL. Add water or PBS to the edge wells of a 96-well plate to prevent evaporation, which can affect monoclonal growth. After picking clones, first expand in a 48-well plate, freeze half, and extract gDNA from the other half for validation. If the homology arms are long or the cell condition is poor, resistance selection can be added first for enrichment, but for point mutation editing, increased resistance cannot be used as the sole evidence and must ultimately be confirmed by sequencing.