Efficient regulation of transgene Would greatly facilitate the analysis of gene function in biological systems for basic research and clinical applications. The tetracycline-regulatable system (TRS) has proven to be a promising tool for such purposes. Despite their widespread application, a number of challenges are still associated with the use of TRS, including clonal variability in the regulation and copy number. We have recently constructed a novel human artificial chromosome (HAC) called 21 Delta qHAC. By housing a TRS-based DNA-PKcs expression cassette in this HAC, we were able to circumvent the problems associated with conventional TRS-based vectors. We achieved tight control of DNA-PKcs expression and rescued the radiosensitive phenotype of DNA-PKcs-deficient CHO cells. The combined use of HAC and the TRS serves as a model for controllable and fixed copy number expression vectors. Our study also demonstrates the suitability of the HAC to accommodate multi-subunit constructs such as that of the TRS. (c) 2005 Elsevier Inc. All rights reserved.