2 39 3 39 dideoxypyrimidine nucleosides for example azt azt p azt azt p dda azt p ddi azddclu azddmec azddmec n4 oh azddmec n4me azt p cye dda azddetu cs 85 azddu cs 87 azddc cs 91 azddfc azddbru and azddiu the class comprising 3 39 halopyrimidine dideoxynucleosides for example 3 fddclu 3 fddu 3 fddt 3 fddbru and 3 fddetu the class comprising 2 39 3 39 didehydro 2 39 3 39 dideoxynucleosides d4 nucleosides for example d4t d4c d4mec and d4a the class comprising 2 39 3 39 unsubstituted dideoxypyrimidine nucleosides for example 5 f ddc ddc and ddt the class comprising 2 39 3 39 unsubstituted dideoxypurines nucleosides for example dda dddapr diaminopurine ddg ddi and ddmea n6 methyl and the class comprising sugar substituted dideoxypurine nucleosides for example 3 n 3 dddapr 3 n 3 ddg 3 fdddapr 3 fddg 3 fddaraa and 3 fdda wherein me is methyl et is ethyl and cyet is cyanoethyl other suitable nucleoside analogues may be antiviral agents like acyclovir or gancyclovir dhpg or other analogues as described below preferred dideoxy derivatives are those used in the treatment of aids including 3 39 azido 3 39 deoxythymidine azidothymidine or azt 2 39 3 39 dideoxythymidine ddt 2 39 3 39 dideoxycytidine ddc 2 39 3 39 dideoxyadenosine dda and 2 39 3 39 dideoxyguanosine ddg azt ddi dda ddt and ddc are most preferred analogues at present the didehydropyrimidines as well as carbovir a carbocyclic 2 39 3 39 didehydroguanosine.