DNA is the universal primary target for ionizing radiation; however, the cellular response is highly diversified not only by differential DNA repair ability. The monitoring system for the ionizing radiation-inflicted DNA damage consists of 3 apparently independently acting enzymes which are activated by DNA breaks: two protein kinases, Atm (ataxia telangiectasia mutated) and DNA-PK (DNA-dependent protein kinase) and a poly(ADP-ribose) polymerase, PARP-1. These 3 enzymes are the source of alarm signals, which affect to various extents DNA repair, progression through the cell cycle and eventually the pathway to cell death. Their functions probably are partly over-lapping. On the side of DNA repair their role consists in recruiting and/or activating the repair enzymes, as well as preventing illegitimate recombination of the damaged sites. A large part of the nuclear signalling pathway, including the integrating role of Tp53 has been revealed. Two main signalling pathways start at the plasma membrane: the MAPK/ERK (mitogen and extracellular signal regulated protein kinase family) "survival pathway" and the SAPK/JNK (stress-activated protein kinase/c-Jun N-terminal kinase) "cell death pathway". The balance between them is likely to determine the cell’s fate.
When DNA break rejoining is impaired, the cell is unconditionally radiation sensitive. The fate of a repair-competent cell is determined by the time factor: the cell cycle arrest should be long enough to ensure the completion of repair. Incomplete repair or misrepair may be tolerated, when generation of the death signal is prevented. So, the character and timing of the signals are, to a large part, responsible for the cellular intrinsic radiation sensitivity and depend on the characteristics of the cellular signalling web.