PAGE 7 OF 8 molecules. Dimer formation is thus much more likely then formation of trimers and tetramers. CONCLUSIONS We have presented a simple procedure to speciﬁcally end- join two biotinylated nucleic acid molecules A and B using STV as an intermediary. The method avoids the time- consuming optimization usually required for ligation reac- tions when long molecules are involved or when joining DNA to RNA or RNA to RNA. The method presented here appears to be insensitive to the length of the mole- cules (up to at least 20 kb long DNA) or to whether the molecules are DNA or RNA. The motivation to use STV to end-join two nucleic acids was based on the fact that the STV–biotin linkage is the strongest noncovalent intermo- lecular bond known, but also because it is relatively straightforward to enzymatically biotinylate DNA and RNA molecules. We note that this procedure could in principle make use of any other pair of strongly binding antigens, such as digoxygenin and antidigoxygenin or ﬂuorescein and antiﬂuorescein (21,24–26). The use of such antigen-pairs could also be useful in typical single molecule applications, such as magnetic or optical twee- zers, where also the other ends of the molecules A and B will be attached to either a bead or a surface (46–48). Conveniently, such an approach will automatically ﬁlter out the A+B product (the A+STV and B+STV alone will not tether between both surfaces), which will increase the overall yield of the protocol even more. The binding protocol presented here can be performed easily and rapidly in a standard biochemical buﬀer, and no special chemicals and subsequent puriﬁcations are required. It is therefore an ideal choice to create long nucleic acid con- structs that may ﬁnd many applications in bionanotech- nology and single-molecule experiments. 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