The ideal cell lineage marker is one that can be visualized in living tissues without perturbing development. Exogenously applied dyes are very useful, but in many instances there would be advantages to an endogenously expressed marker. Green fluorescent protein (GFP) appears to have the potential to be ideal for this purpose, and with its advent, lineage analysis is entering a new phase in which cells can now be followed in real time in living embryos. GFP is a 27-kDa protein found in the jellyfishAequorea victoriathat absorbs blue light and emits green light. It has the valuable property that the formation of the chromophore is an autocatalytic event that requires no cofactors. Furthermore, the cDNA that encodes GFP has been cloned, and thus GFP can be expressed and will become fluorescent in any organism or cell type. In this chapter, we describe the properties of GFP and the different mutants available to study cell lineages in different organisms. We will mainly concentrate on their application to the analysis of cell lineages in frogs and mice.
