Recent studies provided experimental in vitro evidence that O3 at low concentrations does not induce cytokinetic effects on tumour cells or reduce drug cytotoxicity; the proliferation and motility of human cervical and breast cancer cells were not affected by O3 administration [213], and when O3 was combined with an anti-tumour agent (tamoxifen) acting through induction of oxidative stress, no reduction in drug cytotoxicity was observed as Nrf2 was not overstimulated [214]. This evidence concerns the gene NFE2L2 and breast cancer.