The proliferation of eukaryotic cells is driven by a process called the cell cycle. Proper regulation of this process, leading to orderly execution of sequential steps within the cycle, ensures normal development and homeostasis of the organism. On the other hand, perturbations of the cell cycle are frequently attributed to cancer cells. Mechanisms that ensure the order and fidelity of events in the cell cycle are called checkpoints. The checkpoints induced by damaged DNA delay the cell cycle progression, providing more time for repair of lesion before DNA replication and segregation. The DNA damage-induced checkpoints can be recognized as signal transduction pathways that communicate information between DNA lesion and components of the cell cycle. Proteins involved in the cell cycle, as well as components of the signal transduction pathways communicating with the cell cycle, are frequently products of oncogenes and tumor suppressor genes. Malfunction of these genes plays a critical role in the development of human cancers. The key component in the checkpoint machinery is tumor suppressor gene p53, involved in either regulation of the cell cycle progression (e.g. Gl arrest of cells treated with DNA damaging factor) or activation of programmed cell death (apoptosis). It is postulated that p53 protein is activated by DNA damage detectors. One of the candidates for this role is DNA-dependent protein kinase (DNA-PK) which recognizes DNA strand breaks and phosphorylates p53 protein.