The long-term goal of this proposal is to understand ATM and ATR kinase signaling at DNA replication forks. While ATM kinase is activated by DNA double strand breaks, ATR kinase is induced by single-stranded (ssDNA) gaps. However, crosstalk exists between the pathways and ATM and ATR phosphorylate an overlapping set of substrates. To identify indispensable ATM kinase signaling we used the ATM kinase inhibitors KU55933 and KU60019 to transiently inhibit ATM kinase activity in cells. Using this innovative approach we showed that the consequences of acute ATM kinase inhibition and ATM protein disruption are distinct. Here we show that acute ATM kinase inhibition arrests DNA synthesis. This is surprising since irradiated cells that express no ATM protein do not arrest DNA synthesis due to a defect in the inhibition of late origin firing. The contribution of chain elongation arrest to the intra-S-phase checkpoint has been difficult to establish since lesions that induce the checkpoint also directly arrest replication forks. While recent evidence indicates that ATR signaling can arrest chain elongation, the role of ATM has not been addressed. We hypothesize that ATM kinase activity promotes the steady progression of replication forks and attenuates ATR kinase activity. We propose that acute ATM kinase inhibition impedes the repair of damaged replication forks causing an accumulation of ssDNA gaps that induce ATR kinase signaling and the intra-S-phase checkpoint. This challenges the paradigm that ATM kinase disruption disables the intra-S-phase checkpoint. Here we will combine the use of KU55933 and KU60019 as sharp tools to inhibit ATM kinase signaling with single DNA fiber-based technology that allows the visualization of multiple origins and individual replication forks emerging from those origins. We will undertake the first investigation of origin density and replication fork velocity in cells following acute ATM kinase inhibition and ATM protein disruption.