The DDR integrates the regulation of cell cycle progression with DNA repair, which allows time for repair and prevents DNA damage being permanently passed to daughter cells [1-3]. There are three checkpoints (G1/S, S phase, and G2/M) where the cell cycle may arrest in response to DNA damage (see the figure below) [1, 2, 4]. Accordingly, the specific proteins associated with each checkpoint are being targeted by small molecule inhibitors. These DDR-targeting agents aim to either maximise DNA damage in the G1 and S phases of the cell cycle or to prevent repair in G2, in order to ensure that the maximum amount of DNA damage is taken into mitosis.
Figure 6: Cell Cycle and DDR targeting 
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