Figure 1.

Every 3617 cell contains at least one MMTV array, and these arrays are visible by GFP-GR in a subset of cells. (a) A low magnification projection image constructed from 16 confocal sections, .28 μm apart, showing 3617 cells that were removed from tetracycline for 16 h and then treated with 100 nM dexamethasone for 1.5 h. Note that the GFP-GR is concentrated in the nucleus of each cell. At least two cells exhibit moderately large MMTV arrays, indicated by the white circles (see McNally et al. [2000] and Figs. 2 and 7 here for a demonstration that these bright structures correspond to the MMTV array). Other nuclei do not reveal obvious array structures, although several exhibit bright structures near nucleoli that may correspond to arrays but cannot be scored definitively at this low magnification. (b) Control DNA FISH with a probe specific for the MMTV array on mouse C127 cells, the grandparent of 3617 that lacks the MMTV array. No DNA FISH signal is detected. Contrast has been amplified in this panel to permit visualization of background staining in the nucleus. (c) DNA FISH with a probe specific for the MMTV array on 3617 cells. Each nucleus shows a distinct FISH signal, indicating that every cell contains at least one copy of the MMTV array. (Sometimes nuclei are observed with two such signals; data not shown.) Bar, 10 μm.