Figure 5: Quantification of 53BP1 foci in tissues from repair-proficient and -deficient mice exposed to fractionated low-dose radiation. The numbers of 53BP1 foci per nucleus in bronchiolar and alveolar cells (lung), cardiomyocytes (heart), and cortical neurons (brain) were counted 24 and 72 h after fractionated low-dose radiation (10× and 20× fractions, 10 mGy) in repair-proficient (ATM+/+), questionable repairproficient (ATM+/-) and repair-deficient (ATM-/-, SCID) mouse strains. Data presented are means from four different samples ± standard error. (*denotes a significant difference compared to non-irradiated control tissue.)