In the fungus Aspergillus nidulans the Dbf4-dependent kinase (DDK) is composed of regulatory and catalytic subunits encoded by nimODbf4 and cdc7, respectively. nimODbf4 associates with cdc7p, activating the kinase and escorting it to origins of replication where it triggers DNA synthesis. A nimO mutation, nimO18, confers temperature sensitive cell cycle arrest in late G1, and at permissive temperature exhibits profound sensitivity to agents that cause double strand breaks (DSBs), such as Diepoxyoctane (DEO). We discovered a novel suppressor of nimO18, called snoA (suppressor-of-nimO). Intriguingly, loss of snoA function substantially alleviates the ts-lethal and DNA damage-sensitivities nimO18, indicating that snoA may act normally to inhibit nimO function and thereby restrain DNA synthesis in response to DNA damage. In a search for other DNA damage responses that may be inhibited by snoA, we discovered that DEO-sensitive defects in g-H2AX (H2AX-S129A) and Rad953BP1 (¿Rad9) are also partially relieved by loss of snoA function. g-H2AX and Rad953BP1 are components of an ATM-dependent DNA damage response (DDR) pathway that responds to DSBs. In this study, we are assessed epistasis relationships between H2AX, Rad9 and snoA and determined that snoA antagonizes the DNA damage checkpoint pathway controlled by Rad953BP1 and g-H2AX.