Each chromosome occupies its own-specific space called a 'territory' within the interphase nucleus, and the arrangement of chromosome territories (CTs) is important in epigenetic mechanisms. The molecular mechanism to determine the positioning of CFs, however, remains unknown. On the other hand, dioxin is known to be the typical environmental pollutant that affects a wide Variety of biological events in many species. Here, we show that dioxin enlarges the minimum distance between chromosome 12 and chromosome 16 territories in human preadipocyte cells, and the alteration of chromosome positioning is canceled by an aryl hydrocarbon receptor (AhR) antagonist alpha-naphthoflavone. Thus. AhR may be a key molecule to regulate chromosome positioning. Our results suggest a novel effect of dioxin toxicity, and demonstrate a Clue to reveal the novel molecular mechanism for the arrangement of Us. (C) 2008 Elsevier Inc. All rights reserved.