In a separate plate, the mixture of CP and 1% S9 was preincubated at 37C for 30min, which is the same procedure used in the DDR assay, and 16.7 l of CP-S9 mixture was transferred to the 96-well plate to set the final concentration of S9 at 0.1%. overall toxicity testing framework developed by the committee focuses on four major components: chemical characterisation, toxicity Amezinium methylsulfate pathways and targeted testing, doseCresponse and extrapolation modelling and population-based and human exposure data. With regards to the conventional genotoxicity assays, little attention has been paid to the doseCresponse pattern and toxicity pathways activated by genotoxic agents. Recently, a mechanistic understanding and quantitative analysis of genotoxic agents were highlighted in order to determine acceptable exposure levels in humans (2C6). The use of a comprehensive set of tests to identify the pathways affected in the presence of genotoxic agents would provide much stronger, mechanistically based, predictive tools for human health risk assessment. For this purpose, the US Tox21 program adopted a DNA damage response Amezinium methylsulfate (DDR) assay utilising isogenic chicken DT40 cell lines that broadly probed biological targets, pathways and mechanisms in relation to genotoxicity and/or cytotoxicity endpoints for a large number of chemicals (7,8). The reverse genetic approach provides a powerful method for studying gene function and regulation. DT40 cells originated from a chicken B-lymphocyte line TNFSF13B derived from an avian leucosis virus-induced bursal lymphoma isolated in 1985 (9). We established a multiwell-plate-based method that makes use of the DT40 isogenic cell line and its dozens of available mutants knocked out in DNA repair and cell cycle pathways Amezinium methylsulfate (10C12). This assay, Amezinium methylsulfate which is based on increased cytotoxicity in DNA repair-deficient DT40 mutants versus the parental DT40 cells, is a rapid and simple method to evaluate the genotoxicity of xenobiotics and is suitable for high throughput screening (8). In order to screen a broader range of chemicals, the current DT40 cell-based DDR assay needs to incorporate metabolic activation because some xenobiotics show genotoxic potential only after metabolic activation. In this study, we applied a metabolic activation system using S9 to the DT40 cell-based DDR assay. We first utilised a cell-washing method for the metabolic activation system. The washing method is an established procedure for metabolic activation in the genotoxicity study; however, this process may introduce physical stress to the cells from centrifugation and loss of cells by media change. In particular, DT40 cells are very sensitive to various environmental stressors, such as pipetting pressure and temperature (11); therefore, it is better to avoid unnecessary stress derived from washing, centrifugation and handling errors. Furthermore, the washing method is not practical to screen for many chemicals particularly in the high-throughput format. We decided to incorporate the S9 metabolic activation system using a convenient method that requires only the addition of the reagents in the DT40 cell-based DDR analysis. Consequently, DT40 cells need to be cultivated in the presence of S9 fractions. However, cytochrome P450 metabolises lipids that make up S9 microsomes and result in the formation of toxic microsomal lipid peroxides (13,14). It is also known that cytochrome P450, in the absence of substrates, cycle electrons and could produce reactive oxygen species (15). Using preincubation method, we investigated the ability of cyclophosphamide (CP), a genotoxin requiring metabolic activation (16), to induce differential cytotoxicity across the different DNA repair-deficient DT40 cell lines. Materials and methods DT40 cell culture and maintenance Fetal bovine serum (FBS) and penicillin/streptomycin were obtained from Atlanta Biologicals (Norcross, GA, USA) and Sigma-Aldrich (St. Louis, MO, USA), respectively. RPMI 1640 culture medium (+glutamine, Cphenol red) and chicken serum (CS) were acquired from Life Technologies (Grand Island, NY, USA). FBS and CS were heat inactivated at 56C for 30min. DT40 cells were maintained as described in our previous report (11). The list of twenty DT40 isogenic mutants used in this study is shown in Table 1 and Supplementary.