Gene amplification plays an important role in tumor progression and development of drug resistance, which manifested cytogenetically as double minutes (DMs) or 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, two model systems harboring HSRs and DMs were used, respectively. In this study, we, for the first time, demonstrate a novel specific role for NHEJ in the formation of DMs, but not HSRs, in MTX-resistant cells, and that NHEJ might be a potential target for the treatment of MTX-resistant colon cancer. (By Dr. Songbin Fu, http://jmg.bmj.com/content/early/2014/12/23/jmedgenet-2014-102703 )