Graphene oxide (GO) has attracted enormous interests due to its extraordinary properties. Recent studies have confirmed the cytotoxicity of GO, we further investigate its mutagenic potential in this study. The results showed that GO interfered with DNA replication and induced mutagenesis at molecular level. GO treatments at concentrations of 10 and 100 mg/mL altered gene expression patterns at cellular level, and 101 differentially expressed genes mediated DNA-damage control, cell apoptosis, cell cycle, and metabolism. Intravenous injection of GO at 4 mg/kg for 5 consecutive days clearly induced formation of micronucleated polychromic erythrocytes in mice, and its mutagenesis potential appeared to be comparable to cyclophosphamide, a classic mutagen. In conclusion, GO can induce mutagenesis both in vitro and in vivo, thus extra consideration is required for its biomedical applications. G raphene, firstly isolated from graphite in 2004 1 , is a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice. Due to the unique physicochemical properties, high surface area, excellent thermal conductivity, electric conductivity, and strong mechanical strength, graphene and graphene oxide (GO) have shown great promise in many applications, such as electronics, energy storage and conversion, mechanics, and biotechnologies [2][3][4][5][6] . Recently, many studies reported that GO has outstanding potentials in the field of biomedicine. GO and PEGylated GO exhibit certain advantages in vitro and in vivo drug delivery, such as high drug loading efficiency, controlled drug release, tumor-targeting drug delivery, and reversal effect against cancer drug resistance [7][8][9][10] . In addition, GO has strong optical absorbance in the near-infrared (NIR) region, thus is suitable for the photothermal therapy [11][12][13] . Now, it is possible to manufacture high-quality GO in large scale quantities 14,15 , and its industry production is increasing exponentially. Together with its potential applications in the biomedical field, the biosafety of GO is of critical importance. Many investigations have paid attentions to its biocompatibilty [16][17][18][19] . At a concentration approximate to 50 mg/mL or higher, GO begins to show the toxicity against erythrocytes, fibroblasts, and PC12 cells. It can induce cell apoptosis, hemolysis, and oxidative stress 16,18,19 . Surface chemical modification, such as PEGylation, is likely to improve the biocompatibility of GO 20,21 . However, the chemical bonds linking GO with modified polymer can be broken down in vivo, thus surface-modified GO can also induce in vivo toxicity.Several investigations have reported that treatments with carbon nanomaterials, such as nanodiamonds and multiwalled carbon nanotubes, can elevate the expression of p53, MOGG-1, and Rad51, which reflect the chromosomal DNA damage 22,23 . However, it is not clear whether this DNA damage induced by carbon nanomaterials can cause mutagenesis. GO, due to its unique nanosheet structure, can interact wi...