Nuclear DNA is undoubtedly the most precious component of a cell. It is not surprising therefore that any kind of damage that introduces a discontinuity in the DNA double helix elicits a prompt cellular reaction. The DNA damage response (DDR) provides an intrinsic biological barrier against the duplication and partitioning of damaged DNA into daughter cells and impedes the propagation of corrupted genetic information. When maintenance of genome integrity fails, it might lead to programmed cell death (apoptosis), or genomic instability (GIN), which in turn can cause cell transformation and oncogenesis. Among the Serine and Threonine specific kinases, a number of them is involved in the processes that play a significant role in the DDR. For example, Ataxia telangiectasia mutated (ATM) kinase recognizes and signals to double-strand breaks (DSB), which are among the most critical lesions in chromosomal DNA. ATM is present in the nucleus as an inactive dimer or oligomer, and is activated in response to DSBs in a process that involves autophosphorylation. This causes a dissociation of the dimer to form active monomeric forms, which are able to initiate the phosphorylation of many intermediates, such as p53 and the checkpoint kinase Chk2, which are involved in DNA repair and cell-cycle control. Similar to ATM, the ataxia-telangiectasia and Rad3-related (ATR) protein and the DNA-activated protein kinase (DNA-PK) play an important role in responding to agents and extracellular stress that threaten the DNA replication process. Interestingly, a normal and robust checkpoint pathway is thought to be a mechanism of resistance to chemotherapy. As a result, ATR-Chk1 pathway components are considered promising therapeutic targets. In particular, inhibition of ATR-Chk1 pathway components could potentially enhance the effectiveness of replication inhibitors.