There are two ways the cell can fix the double stranded break (DSB): Non-homologous End Joining (NHEJ), which is sloppy, and Homology Directed DNA Repair (HDR), which is cleaner but limited to two stages of the cell cycle.

This article describes the challenge.

“Given that HDR is restricted to the S and G2 phases of the cell cycle, only present in dividing cells, this approach might not be suitable for editing and repairing the genome in non-dividing cells.”

Here is the original paper that the above website sites as a source.