DNA strand breaks in hepatic tissue. DNA strand breaks in liver tissue from mice exposed to 0, 18, 54 or 162 μg of NanoTiO2 (A) or Printex 90 (B) or 0 or 486 μg of Indoor-R or Indoor-NanoTiO2 (C). The statistical analysis for Printex 90 has been described previously . The increased level of DNA strand breaks of Indoor-R is considered an artifact due to 1) a low plate control and the normalization procedure and 2) the unusual kinetic of DNA damage over time. *, **, ***: Statistically significant compared to control mice at the 0.5, 0.01 and 0.001 level, respectively.