The different DNA damage repair pathways like homologous recombination (HR) and non-homologous end joining (NHEJ) have been linked to the variation of radiosensitivity throughout the cell cycle. However, no attempts have been made to test the various hypotheses derived from these studies in a quantitative way e.g. by using modelling approaches. Here we present the first modelling approach that allows predicting the cell cycle dependent radiosensitivity of repair proficient as well as of repair deficient cell lines after photon irradiation based on a small set of parameters and assumptions.