Quantum dots (QDs) have received a great amount of interest for use as fluorescent labels in biological applications. QDs are brightly fluorescent and very photostable, satisfying even imaging applications that require single molecule detection at high repetition rates over long periods of time (minutes to hours). There are by now numerous methods for conferring biospecificity and function including cell membrane permeability to QDs. A particular convenient method of conferring membrane penetrating ability and in some cases also biospecificity has been to couple biotinylated protein transduction domains to streptavidin-conjugated QDs. This method, which is easily customizable and requires minimal custom conjugation, is suitable for long-term in vitro and in vivo cell tracking imaging applications.