How do you favor HDR over NHEJ for precise CRISPR editing?

Strategies to enhance HDR include: cell cycle synchronization (HDR active in S/G2, NHEJ throughout), using small molecule inhibitors (SCR7, NU7441 inhibit NHEJ; RS-1 enhances HDR), optimizing donor template design (ssODN for small edits, plasmid or AAV for larger insertions, symmetric vs asymmetric homology arms), Cas9 variant selection (Cas9 nickases with paired guides reduce NHEJ), controlling Cas9 expression timing and duration, and choosing cell types with higher HDR activity. Delivery method matters - RNP delivery provides transient Cas9, reducing prolonged cutting. Despite optimization, HDR typically remains less efficient than NHEJ in most cell types.