DNA damage expressed as percentage of tail DNA following exposure of the human liver microtissue to single or repeated exposure of engineered NMs. The tissues were exposed to cell medium (control) or NMs on one or repeated occasions. Values represent mean ± SEM (n = 3) unless otherwise stated. Significance indicated by * = p < 0.05 and ** = p < 0.005, when material treatments are compared to the control. Ψ = p < 0.05 and ΨΨ p < 0.005 is representative of significant difference between values signifying absence and presence of Fpg enzyme at each given concentration. Φ = p < 0.05 and ΦΦ p < 0.005 is representative of significant difference between single and repeated exposures. £ signifies n = 2 for that particular treatment. A) ZnO - NM 110 B) Ag – NM 300 C) MWCNT - NM 400 D) TiO2 – NRCWE 002. SING – single exposure, REP – repeated exposure.