Green fluorescent protein (GFP) and its homologs are widely used as fluorescent markers of gene expression and for determination of protein localization and motility in living cells. In particular, based on GFP and GFP-like proteins a number of techniques have been developed that can be used either to estimate protein mobility in living cells, or to introduce a distinctive fluorescent signal in order to track the movement of labeled molecules directly. Considerable progress in the development of such technologies in the last two or three years motivates us to reevaluate the present scope of biotechnological instruments in studies of protein movement in cells.