Improving the precision of gene correction in human embryos by YEE-BE3. (A) Schematic of repairing HBB −28 (A>G) in cloned human embryos by YEE-BE3 and gRNA-1. Firstly, cloned HBB −28 (A>G) mutant homozygous human embryos were generated by fusing skin fibroblast cell from the patient with in vitro matured oocytes. And YEE-BE3 mRNA and gRNA mixture was injected after removing PB1. And 1 h later, the injected oocytes were fused with skin fibroblast cells. Then the fused embryos were activated and cultured for another 48 h. Single blastomere was isolated and MDA amplified. Then HBB site was amplified and sequenced. PB1, the 1st polar body. PN, pronucleus. ZP, zonapellucida. (B) Summary of base editing-mediated point mutation repairing by YEE-BE3 in cloned human embryos. The numbers of homozygous mutant blastomere (G−28G−25/G−28G−25), heterozygous blastomeres (A−28G−25/G−28G−25), and wild-type blastomeres (A−28G−25/A−28G−25) were calculated. #, 4 embryos did not develop into 2-cell stage. *, HBB site failed to be amplified by PCR. (C) Sanger sequencing to detect successful repairing by YEE-BE3 in each blastomere. Representative sequencing chromatographs of homozygous mutant blastomeres, heterozygous blastomeres and wild-type blastomeres