Rapid movement of DHFR BAC transgenes along the transgene array axis implies chromatin conformational changes over a several second time scale. Live-cell imaging of DHFR BAC B9 cell line with ∼1-s time intervals (Video 4). Two examples (left and right) show GFP spots moving relative to each other (arrowhead) along the direction of the BAC transgene array chromosome axis. Video 4 shows most GFP spots are in rapid motion relative to each other on a time scale far shorter than the estimated 10 min required for complete transcript synthesis, implying the absence of stable attachments of transgenes to surrounding nuclear structures during transcription. Bar, 2 µm.