of signaling important for the maintenance, and not initiation, of the G1 checkpoint (through p53 inhibition) since there was no difference at earlier time points following IR (74). Finally, evidence suggests that Wip1 has a function in the intra S-phase cell cycle checkpoint after stress. Lu et al. showed that Wip1 plays a role in regulating the intra-S phase after IR and UV radiation in cells either overexpressing Wip1 or that have Wip1 knocked down by siRNA (75). Therefore, Wip1 helps to return the cell to normal cell cycle progression after stress by negatively regulating pathways involved G2/M, G1/S, and the intra-S phase checkpoints.
