A single-domain antibody (sdAb, also called Nanobody) is an antibody fragment consisting of a single monomeric variable antibody domain. Like a whole antibody, it is able to bind selectively to a specific antigen. With a molecular weight of only 12–15 kDA, single-domain antibodies are much smaller than common antibodies (150–160 kDa). They have been shown to be just as specific as a regular antibody and in some cases more robust. They are being researched for multiple pharmaceutical applications in in vivo imaging and targeted therapy. eg. the fusion of a fluorescent protein to a single-domain antibody can be used to trace targets in different compartments of living cells. They can therefore increase the possibilities of live cell microscopy and will enable novel functional studies.