Gene knock-ins enable the precise insertion of new genetic information at specific genomic locations, a capability essential for applications ranging from reporter gene integration to therapeutic gene correction. Unlike knockouts, which simply disrupt genes, knock-ins require providing a DNA donor template containing the desired sequence flanked by homology arms matching the target locus. When CRISPR-Cas9 creates a double-strand break at the target site, the cell's homology-directed repair machinery uses the donor template to incorporate the new sequence.
The efficiency of gene knock-in has historically been a limiting factor, as homology-directed repair occurs at much lower rates than non-homologous end joining in most cell types. Companies like Inscripta have developed technologies to improve knock-in efficiency, while researchers have explored strategies including cell cycle synchronization, small molecule HDR enhancers, and modified Cas9 variants that bias repair toward the templated pathway. ArsenalBio and other cell therapy companies have invested heavily in optimizing knock-in protocols for T cell engineering, where site-specific integration of chimeric antigen receptors and other therapeutic transgenes can improve the safety and consistency of engineered cell products.
Large-sequence knock-ins are pushing the boundaries of what genome editing can achieve. Recent advances have enabled the insertion of multi-kilobase constructs including entire gene expression cassettes at defined genomic safe harbors. This capability is critical for gene therapy approaches that aim to provide functional copies of disease genes, as well as for synthetic biology applications that require precise genomic integration of multi-gene pathways. Techniques like PASTE (Programmable Addition via Site-specific Targeting Elements), which combines CRISPR with serine integrases, have demonstrated efficient large-payload integration without the limitations of traditional HDR-based approaches.