Gene amplification, a key mechanism for oncogene activation and drug resistance in tumour cells, involves the generation and joining of DNA double-strand breaks. Amplified DNA can be carried either on intra-chromosomal arrays or on extra-chromosomal elements (double minutes). We previously showed that, in rodent cells deficient in DNA-PKcs, intra-chromosomal amplification is significantly enhanced. In the present work, we studied gene amplification in human HeLa cell lines in which the expression of the DNA-PKcs gene was constitutively inhibited by shRNAs. These cell lines showed an increased sensitivity to ionizing radiations, an enhanced frequency of chromosomal aberrations and an increased rate of occurrence of methotrexate resistant colonies compared to the control cell lines (6-18 times). The main mechanism of resistance to methotrexate was extra-chromosomal amplification of the dihydrofolate reductase gene. These results indicate that, in human cells, inhibition of DNA-PKcs gene expression favours gene amplification occurring via the production of double minutes. In addition, they show that cell lines constitutively expressing shRNAs are good model systems to study the role of specific functions in gene amplification.