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However, the sensitivity of this assay is limited because it detects mutations occurring only in the <jats:italic>TK<\/jats:italic> gene but not any other genes. Moreover, the limited sensitivity of the conventional <jats:italic>TK<\/jats:italic> assay is caused by the usage of DNA repair\u2010proficient <jats:italic>wild\u2010type<\/jats:italic> cells, which are capable of accurately repairing DNA damage induced by chemicals. Mutagenic chemicals produce a variety of DNA lesions, including base lesions, sugar damage, crosslinks, and strand breaks. Base damage causes point mutations and is repaired by the base excision repair (BER) and nucleotide excision repair (NER) pathways. To increase the sensitivity of <jats:italic>TK<\/jats:italic> assay, we simultaneously disrupted two genes encoding <jats:italic>XRCC1<\/jats:italic>, an important BER factor, and XPA, which is essential for NER, generating <jats:italic>XRCC1<\/jats:italic><jats:sup>\n<jats:italic>\u2212\/\u2212<\/jats:italic><\/jats:sup><jats:italic>\/XPA<\/jats:italic><jats:sup>\n<jats:italic>\u2212\/\u2212<\/jats:italic><\/jats:sup> cells from TK6 cells. We measured the mutation frequency induced by four typical mutagenic agents, methyl methane sulfonate (MMS), cis\u2010diamminedichloro\u2010platinum(II) (cisplatin, CDDP), mitomycin\u2010C (MMC), and cyclophosphamide (CP) by the conventional <jats:italic>TK<\/jats:italic> assay using <jats:italic>wild\u2010type<\/jats:italic> TK6 cells and also by the <jats:italic>TK<\/jats:italic> assay using <jats:italic>XRCC1<\/jats:italic><jats:sup>\n<jats:italic>\u2212\/\u2212<\/jats:italic><\/jats:sup><jats:italic>\/XPA<\/jats:italic><jats:sup>\n<jats:italic>\u2212\/\u2212<\/jats:italic><\/jats:sup> cells. The usage of <jats:italic>XRCC1<\/jats:italic><jats:sup>\n<jats:italic>\u2212\/\u2212<\/jats:italic><\/jats:sup><jats:italic>\/XPA<\/jats:italic><jats:sup>\n<jats:italic>\u2212\/\u2212<\/jats:italic><\/jats:sup> cells increased the sensitivity of detecting the mutagenicity by 8.6 times for MMC, 8.5 times for CDDP, and 2.6 times for MMS in comparison with the conventional <jats:italic>TK<\/jats:italic> assay. In conclusion, the usage of <jats:italic>XRCC1<\/jats:italic><jats:sup>\n<jats:italic>\u2212\/\u2212<\/jats:italic><\/jats:sup><jats:italic>\/XPA<\/jats:italic><jats:sup>\n<jats:italic>\u2212\/\u2212<\/jats:italic><\/jats:sup> cells will significantly improve <jats:italic>TK<\/jats:italic> assay.<\/jats:p>","DOI":"10.1002\/em.22371","type":"journal-article","created":{"date-parts":[[2020,4,3]],"date-time":"2020-04-03T16:28:21Z","timestamp":1585931301000},"page":"602-610","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":10,"title":["Enhancing the sensitivity of the thymidine kinase assay by using <scp>DNA<\/scp> repair\u2010deficient human <scp>TK6<\/scp> cells"],"prefix":"10.1002","volume":"61","author":[{"given":"Mahmoud Abdelghany","family":"Ibrahim","sequence":"first","affiliation":[{"name":"Department of Radiation Genetics Kyoto University, Graduate School of Medicine  Kyoto 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