ATR kinase is a key regulator of checkpoint responses to incompletely replicated and damaged DNA. Without this checkpoint, cells will enter mitosis prematurely, likely resulting in cell death. The invention provides methods and reagents to either block or activate the activation of the ATR kinase checkpoint, through, for example, either blocking or activating the expression of an ATR activator TopBP1. The invention also provides screening methods to identify additional ToBP1 inhibitors or activators that may be used to modulate the activity of the ATR checkpoint.