Classical fate mapping approaches in any embryo typically involve labeling of single cells with nondiffusible dyes by pressure or iontophoretic microinjection followed by tracing the fate(s) of those cells and their descendants throughout development. The limitation of any approach that involves microinjection as a means of cell labeling is that, as development progresses, cells typically decrease in size and become less accessible to mechanical manipulation. The approach presented here utilizes local activation of a photoactivatable (caged) dye that becomes fluorescent when illuminated with specific wavelengths of ultraviolet light (1 ).
