3.2.4. Impact of the cell cycle on chromatin organization and DNAdamage responseThe previous sections concern mostly chromatin organizationin quiescent cells and, therefore, the radiation response of nor-mal tissues and non-proliferating tumors. Cell cycle impacts onboth the relative contribution of the DNA damage repair pathwaysand the level of chromatin condensation: (1) HR and alterna-tive NHEJ pathways become preponderant in G2/M cells (Iliakis,2009); (2) the average number of DSB induced per Gy per humandiploid cell is 80 in S cells (most decondensed chromatin) and20 in G2/M (most condensed chromatin) whereas it is about 40in G0/G1 cells (see above; (Frankenberg-Schwager, 1989)). Hence,since DNA break repair pathways are activated by the naturalSSB in replication forks, S phase is long known to be the mostradioresistant cell cycle phase while it is associated to the high-est radiation-induced DSB yield. This conclusion strongly suggeststhat: (1) the number of induced DSB is less predictive of the radi-ation response than and the number of unrepaired DSB ; (2) unlike chemotherapy, and the false Tribondeauand Bergonié’s law, radiotherapy preferentially targets quiescentrather than proliferating cells. Besides, itis noteworthy that severe tissue reactions potentially caused byradiotherapy concern most likely quiescent tissues at lower dose:for example, while intestine cells grow faster that skin fibrob-lasts, dermatitis are observed after radiotherapy of breast cancerbefore a total dose of 40 Gy while radiation-induced rectites areobserved after prostate cancer irradiation that may reach 70 Gy, atleast