2'-Deoxyoligonucleotides with 5-fluorocytosine residues incorporated at specific 
positions of the nucleotide sequence are tools of great potential in the study 
of the catalytic mechanism by which DNA cytosine methyltransferases methylate 
the 5-position of DNA cytosine residues in specific sequence contexts. Chemical 
synthesis of such oligonucleotides is described. Two alternative approaches have 
been developed, one of which proceeds via a fully protected phosphoramidite of 
5-fluoro-4-methylmercapto-2'-deoxyuridine 2, the other via a fully protected 
phosphoramidite of 5-fluoro-2'-deoxycytidine 3. Either building block can be 
used in automated oligonucleotide synthesis applying standard elongation cycles 
and deprotection procedures exclusively. The methylmercapto function of 2 is 
replaced by an amino group in the final ammonia treatment used for cleavage from 
support and base deprotection.