  In this paper we review the applicability of autofluorescent proteins for
single-molecule imaging in biology. The photophysical characteristics of
several mutants of the Green Fluorescent Protein (GFP) and those of DsRed are
compared and critically discussed for their use in cellular biology. The
alternative use of two-photon excitation at the single-molecule level or
Fluorescence Correlation Spectroscopy is envisaged for the study of individual
autofluorescent proteins. Single-molecule experiments performed in live cells
using eGFP and preferably eYFP fusion proteins are reviewed. Finally, the first
use at the single-molecule level of citrine, a more photostable variant of the
eYFP is reported when fused to a receptor for neurotransmitter in live cells.
