ABSTRACT
Fluorescence optical microscopy is one of the most important tools to investigate biological samples and to observe structures in living cells. In particular, the development of confocal (Sheppard and Wilson, 1978) and two-photon excitation (2PE) microscopy (Denk et al., 1990) allowed to perform optical sectioning and the imaging of three-dimensional structures of biological samples (Diaspro, 2001). In recent years, such techniques have gained great advantages from the development of green uorescent proteins (GFPs) a s p robes (C hal e e t a l., 1994; Tsien, 1998) a nd v ariants ( Patterson a nd L ippincott-Schwarz, 2002).
