GFP in IRF sections is stable and active at atmospheric pressure and in vacuum. (A) Sequential images depicting the effect of vacuum pressure on the fluorescent signal recorded from a 200 nm IRF section, beginning at atmospheric pressure (Atm), proceeding stepwise to high vacuum, and returning to atmospheric pressure. (B) Overlaid fluorescent signal for GFP-C1, recorded at 200 Pa, onto the matching scanning electron micrograph recorded in high vacuum. Scale bars—25 μm.