The MSH3 gene is a critical component of the post-replicative DNA mismatch repair (MMR) system that maintains genomic stability. It encodes the MSH3 protein, which serves as a housekeeping protein and is ubiquitously expressed at low levels across a wide range of human tissues, including the colon, rectum, small intestine, brain, and reproductive organs ^[1]. The protein primarily functions by forming a heterodimer with MSH2 to create the MutSβ complex, which is specialized in recognizing and initiating the repair of large insertion-deletion loops (IDLs) and dinucleotide or longer microsatellite repeats. Beyond its canonical MMR role, MSH3 is involved in homologous recombination and the repair of DNA double-strand breaks, contributing to cellular resistance against platinum-based chemotherapeutics ^[2]. Furthermore, MSH3 has been identified as a key genetic modifier of repeat expansion diseases, such as Huntington’s disease and myotonic dystrophy type 1, where its activity paradoxically promotes the somatic expansion of toxic CAG/CTG repeats, thereby influencing disease onset and progression ^[3]. The B6-huMSH3 mouse is a humanized model constructed through gene-editing technology, in which the exon 7 to downstream of exon 24 of mouse Msh3 is replaced with the entire human MSH3 gene plus human MSH3 promoter and downstream region, and the human sequence is inserted in reverse to prevent disruption of the Dhfr gene function. This model can be used for research on Huntington’s disease (HD) and myotonic dystrophy type 1 (DM1), as well as for screening, development, and preclinical evaluation of MSH3-targeted therapeutics.

Serpinb9b is located on chromosome 13 of mice. Nuclease Technology will be used to design sgRNA; Serpinb9b knockout mice will be obtained by applying high-throughput electroporation of fertilized eggs. After sexual maturity, sperm were collected for cryopreservation.