Developing in vivo biophysics by fishing for single molecules.

Single-molecule techniques (SMT) provide the possibility to quantitatively analyze the action of single molecules. SMTs can resolve the distribution of states of an ensemble of molecules, collecting information that is otherwise not accessible by typical ensemble techniques. Until now, the application of SMTs in developmental biology was limited. Several recent studies illustrate the possibility to investigate the behavior of single biological molecules in invertebrates such as Caenorhabditis elegans and transparent embryos of model teleosts. These studies have paved the way for the application of fluorescence-based SMTs, e.g. fluorescence correlation spectroscopy, fluorescent energy transfer, or single particle tracking, in developmental biology. This review aims to define SMTs applicable in developmental biology, and discuss properties of an ideal animal model.