Generally pyrimidine derivatives such as 2-hydroxy-substitutedpyrimidine, 2-mercapto-substituted-pyrimidine and 2-amino-substituted pyrimidine are studied. Pyrimidines have been isolated from the nucleic acid hydrolysates. Pyrimidines are among those molecules that make life possible, have been some of the building blocks of DNA and RNA. Several analogues of pyrimidines have been used as compounds that interfere with the synthesis and functioning of nucleic acids e.g. fluorouracil, which has been used in cancer treatment. Modifications in the base, sugar and phosphate moieties of oligonucleotides and oligonucleotide conjugates have been reported. The subjects of medicinal chemical programmes include approaches to create enhanced affinity and more selective affinity for RNA or duplex structures, the ability to cleave nucleic acid targets, enhanced nuclease stability, cellularuptake and distribution, in vivo tissue distribution, metabolism and clearance.