The incorporation of 5-azacytosine residues into DNA causes potent inhibition of 
DNA (Cytosine-C5) methyltransferases. The synthesis of oligodeoxyribonucleotides 
incorporating single or multiple 5-aza-2'-deoxycytidine residues at precise 
sites was undertaken to generate an array of sequences containing the reactive 
5-azacytosine base as specific target sites for enzymatic methylation. 
Preparation of these modified oligonucleotides requires the use of 
2-(p-nitrophenyl)ethyloxycarbonyl (NPEOC) groups for the protection of exocyclic 
amino functions. These groups are removed under mild conditions, thus avoiding 
conventional protocols that are detrimental to the integrity of the 
5-azacytosine ring.