Expression of GFP by HS-27a cells (A and B) and HS-5 cells (C and D) after 12 days of culture in control media. The image is of viable cells examined under a phase/fluorescent inverted microscope (original magnification × 100), with a direct camera attachment. The same field is photographed under illumination with a 450 to 490 nm light source on the left and a combination of 450 to 490 nm plus incandescent light on the right to simultaneously visualize both GFP-CMV–infected and noninfected cells in confluent cultures.