One-step homozygosity in precise gene editing by an improved CRISPR/Cas9 system

Dear Editor, The CRISPR/Cas9 gene editing method has been suc- cessfully applied to modify genomes in many organisms [1]. However, several critical issues remain unresolved and have become major hurdles for its broad applications [1 ]. First, editing efficiency varies widely at different ge- netic loci and some targeted sites are resistant to editing for unknown reasons. Even for the same gene, editing efficiency differs greatly at different positions. Second, generation of undesirable insertions or deletions （InDels） at the target sites constitutes a major issue in precise genome editing and gene therapy. Third, one-step ho- mozygosity in precise gene editing is extremely rare and is highly desirable for genetic and functional analyses,