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Background Gene amplification is a frequent manifestation of genomic instability that plays a role in tumour progression and development of drug resistance.It is manifested cytogenetically as extrachromosomal double minutes(DMs)or intrachromosomal homogeneously staining regions(HSRs).To better understand the molecular mechanism by which HSRs and DMs are formed and how they relate to the development of methotrexate(MTX)resistance,we used two model systems of MTX-resistant HT-29 colon cancer cell lines harbouring amplified DHFR primarily in(ⅰ)HSRs and(ⅱ)DMs.