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Figure 2 Time course of green fluorescent protein (GFP) expression in rabbit smooth muscle cells (SMCs) transfected in vitro with a recombinant retrovirus containing the gene of GFP. Because GFP is spontaneously fluorescent when observed under ultraviolet light, the SMCs were directly observed under an ultraviolet microscope. After induction of GFP with the addition of doxycycline in the culture medium, a progressive increase in GFP expression was observed, peaking at 12 to 24 h, although no significant expression was observed at baseline (i.e., uninduced conditions). See text for details.